THREAD COUNT If everything else is the same (weave, ply, mill, and type of cotton), higher thread count means a smoother, silkier, more expensive fabric.
Thread count is often referred to with a number like 50s, 80s, 100s, 120s, 140s 160s, etc up to 200s. You would be excused for thinking that 80s = 80 threads per inch, but that's incorrect (and a common misconception in the industry). Rather, these numbers refer to the yarn size. 140s means there are 140 hanks (1 hank = 840 yards) of yarn in one pound.
Regardless, 140s fabric has a higher thread count than 120s and 160s fabric has a higher thread count than 140s and so on. If you ask us what the threadcount of a fabric is, we will probably tell you something like "140s" because that number is easy to understand even if it's not a literal measure of threadcount.
PLY Ply is how many yarns are twisted together to make a single thread. Fabrics can either be two-ply or single ply. Two-ply means that two yarns are twisted together to make a single thread that is then woven into the fabric. (Note that this is not at all like two-ply toilet paper!). Two-ply fabrics are generally superior to single-ply fabrics.
WARP & WEFT Warp threads run vertically. Weft yarns run horizontal. A fabric will often use different types of threads in the warp vs. weft directions.
PULLING IT ALL TOGETHER You might see a fabric's construction described as something like "100/2x100/2". The way to read this is that it has 100s two-ply threads running in both the warp and weft directions.
A more complex fabric might have a construction described as "140/2+70/1x70/1+140/2" This fabric is a 140s two-ply fabric that has 70s single ply threads interwoven to create a texture or pattern.
WHAT DO I REALLY NEED TO KNOW? Dress shirts with high thread count are more expensive to produce and more desirable if you like your shirts silky, soft and thin. Just remember that threadcount is not everything. The weave of the fabric, the ply and the mill that it was produced in are also important factors to consider.
FINAL CAVEAT We hate to come across snobby about threadcount. There's nothing "wrong" with lower thread count fabrics. They can be more affordable and are always great for casual shirts. I have some 80s two-ply pinpoints that I wear frequently. I love that they have a more rugged look and feel to them. Consider them accordingly.