The show standard can be viewed in full here. 
Showing for the Racing Homer is very different than showing for a show breed. Different types of birds have been developed over the years for different types of races, different peoples methods, different race courses, different lengths, etc. Also the achievements of a given bird will qualify it to be put into a class more suited for it; such as the diploma or record classes. This is how we end up with so many different classes of pigeons at a show.
'It would, however, be logical to assume that after so many years of breeding for homing instinct, speed and stamina, that a particular physical type would have evolved to dominate or be predominant, as it were, but that is not so. The racing homer is not a bird of one singular body type, and, again, it is completely understandable, given the wide variety of birds bred, and the isolation of their breedings, in many cases. Some members of the racing fraternity have been breeding from the same basic family of birds for over 50, 60, and even 70 years. Again, this isolation (in the genetic, reproductive sense) tends to guarantee diversity as to shape, size and overall comportment.'
The typical (Californian) show categories are:
Racing homer color
Birds likeliest to produce winners
Record Birds flown in current year (young birds)
and World Class- Your best 3 cocks and 3 hens on a team to be judged as a representation of your best breeding and flying stock.
*this class if frequently won by show racers and has no restrictions on show racers.
The record class is used to break up the long distance, middle, and short distance birds from each other. Since they usually have a type about them that is different from the other birds. The diploma class is used to take the best from these three distances and judge them alone as a higher class of bird. Since they've achieved a great velocity at a distance they may vary from the show standard drastically. A bird that wins the 500 is going to look very different than a 'pretty' bird that just 'came' home from the 500; which is why the race/diploma winners are judged separate.
The color class is where people that have racers with un-ordinary colors on not necessarily 'show quality' birds will find a good competition. Some of the more ordinary un-ordinary colors in homers would be recessive red, browns, opals, many different expressions of grizzle, dilute, indigos, and combinations of these colors together.
Eyesign is just that, its a class for the birds that have the 'best' eye according to eyesign 'experts' as they like to be called. I'm seen numerous times where a show racer will take this class.
Birds likeliest to produce winners. I do not have a lot of experience in this class but it is where you bring your best birds and the judge will look and see which bird he will think will be the best breeder based on different things depending on what show you are at.
The World Class is a category that I've never entered. I've seen it judged and it is just 6 of your best breeders that the judge will judge as a team.
For the last 60 years or so here in the United States, we've developed show racers. Because of these birds, many fanciers have stopped showing birds. They were originally called 'cobra heads' because of the perfect shape of their heads. From these early birds in the 50's came two breeds; the show racer and the American Show racer which has become very different than a normal racer. The first though fits the show standard EXACTLY and has caused a lot of frustration in the show pen. I know several people that just gave up showing their racers because of these birds. This is why we don't just have open classes anymore. You either have the bird on a race sheet or you show it in an un-flown class; or in California you don't show it at a racing show, period (unless there is a show racer section.)
The Racing Pigeon
Although the homing pigeon has been around for thousands of years, the idea of racing them was formed a bit more recently. It all started in Belgium during the early 1800's. Breeders began to develop the racer by mixing select breeds to improve the bird's form, flying, and homing ability. The foundation breeds were the sturdy Smerle and the high-flying Cumulet. The Carrier was later introduced for its long distance capabilities. Other breeds such as the Dragoon, Horseman, Cropper, and Tumbler gave shape to the homers.
The first long distance race was held in Belgium in 1818. Following this, Holland and England quickly became interested. By 1875, they too were breeding and racing their own homing pigeons. In an attempt to improve the breed, the strains of Belgium and England were crossed. It was a trial and error process, but in the end produced a fast, reliable flier. From these, the modern racing pigeon was born.
In the eyes of your average person, the racing pigeon looks no different from the ferals they see in town. It has no fancy crests, muffs, or frills; basically "just a pigeon". It does however have a more developed cere and is built much less dainty than the Rock Dove.
A typical size for homers is somewhere around 15-18 ounces. Size and shape both vary depending on where they came from and what kind of races they're bred for. Some are long and slender, some are short and stubby, and others may be somewhere in between.
Most racers you see are blue or red (often called 'mealy'), but you can find them in just about any other color known to the pigeon world. Along with a rainbow of colors, comes many patterns as well. You'll find bars, checks, grizzles, splashes, and more trekking across the sky towards home.
How To Care For
How much work it takes to care for racing pigeons depends on what you're looking to accomplish. If you just want them to fly around the house, it'll be pretty simple. But if you want to have good quality birds that you can depend on coming home when released, you may want to put in a little extra effort.
All pigeons need the same basic things: clean food and water, and a clean, dry loft they can feel safe and comfortable in.
Food - For racing pigeons, this changes for the different seasons and fanciers. During breeding season you'll want more protein so the babies grow up big and strong. For the racing seasons you'll want something that'll give them more energy. And of course during the winter you may need to adjust your feeding so they stay warm and healthy.
Each person has their own preference for how/what they feed their birds. There are many pigeon mixes to choose from, which you can find online from pigeon suppliers or locally at feed stores. You can also mix your own to have more control over what you're birds get out of their food. Also, most fanciers feed their birds twice a day.
Water - Clean water is the key to good health. Most diseases are passed through the drinking water. Pigeons do drink more during the summer, so be ready for that.
Housing - If you're racing them, a loft is what you'll need. Keep in mind the birds need at LEAST 1 square foot of space per bird. Some form of perches should be provided for them to rest on. It is also best to build the loft to where the roof is not much taller than you are. This makes catching birds much easier. Having wire sections on the loft is good to let them get sunlight and fresh air. Since pigeons make a lot of feather dust, the loft needs to be well ventilated. During the winter or very wet days, you can cover up some of the wire to keep the loft at a comfortable temp. and humidity. And last but not least, keep the loft clean. Letting the droppings and the dust sit in there is not good for their health.
This is the part where a million different methods and opinions come together. I'll try my best to summarize this but it's best to do some research on your own to figure out just how you want to do things with your racers.
There are two racing seasons - Young Bird (YB) and Old Bird (OB).
YB season is usually held in the fall and consists of shorter distance races between 100-300 miles. The only birds that may compete here are those that are banded with seamless bands of the year the season is taking place.
OB season takes place in the spring and are open to any birds a year of age or older. These must also be banded with seamless bands. Certain clubs may have rules on which kind of bands you can race (ex. some may not allow NPA bands, but will allow IF and AU).
A season may last 8 weeks or more, depending on how your club or combine goes about it.
When you first require or wean birds you plan on flying, you'll want to start getting them accustomed to the daily loft life. You want them comfortable around you and to know home is their safe haven. You'll also want them to get trained to a feed call. Every time you feed them, whistle, shake a can, or make some kind of sound so they know that each time they hear that command, it means to come in and eat. This really helps with trapping your birds on race day. Along with this trap training is also important. You can use a settling cage or some other method to get them used to coming in through the trap door.
In order to prepare for the races, fanciers take their birds on training tosses. Road training starts once the young birds have started coming out of the loft and have a good hold on flying. Again, each fancier has his or her own opinion on where to start training. In my opinion, you shouldn't push your birds any more than you feel comfortable with. Your pigeons will tell you if they aren't ready, you just have to pay attention. Starting them out at a small distance (say, 5 miles for example), you release them there a few times and work your way out farther and farther. This allows the birds to get good exercise and practice, and gets them familiar with what to do when taken to an unknown place and released.
And finally race day comes. The night before the race, usually on a Friday, you'll take your birds and your clock to meet the rest of the club. If you're using a manual clock, you'll put countermarks (rubber bands with numbers on them) on their leg and load them in the crates. The number on the countermark is recorded for each bird. If using an electronic clock, you'll scan the chip rings and each is also recorded for the bird it is on. When they arrive home the next day, they're bands will either scan when they cross the landing pad, or you'll have to catch the bird to remove the countermark and put it in the clock. The arrival time is recorded, and going by that and the time released, you are able to calculate the bird's speed in yards per minute. The fastest bird home, wins.
The racing pigeon is known for the fact that it can be the most wild pigeon or the tamest of the tame. It is shaped mentally and physically by its breeder. A good fancier will have his birds tamed to where He can have physical contact with them, without stress on the bird or the fancier. Many have their birds so wild they can't walk into the loft without the birds trying to escape the loft; while others have their birds very tame. I can remember hearing stories (i know this is possible because I had three hens like this when i was 9) about people that had birds land on them after races. This was back before the new e.t.s. and they were able to take the rubbers off the birds and save countless seconds by having their birds tamed to this point. This proves just how a fancier, with enough patience and intelligence, can have his birds to where He can truly enjoy his birds and his birds can enjoy him.