Routes are generally made up of a series of significant points along your path. The GPS will tell you the bearing and distance to the next point in sequence as you navigate along your route. Each point is usually named (in fact, a route is usually just a sequence of waypoints). There are usually at most a few tens of points in a route. (Garmin for instance has 20 routes with up to 30 waypoints each in most of their units.)
Tracks are made up of data gathered automatically from your motion while the GPS is turned ON. They provide a record of where you've been, and when, so you can later determine your path and speed. There may be hundreds or thousands of such points in a track, but they are all anonymous. They don't have names and you can't easily get the location of any particular one.
If you use Garmin's TracBack function, it will turn a track into a route that goes in the opposite direction. This is done by analyzing the path of the track and placing automatically created waypoints at significant turning points in the path. Since there may be a maximum of 30 waypoints (50 in some units) in a route, some track detail may be eliminated in this process.
Actually this description is accurate for all Garmin units except eMap and eTrex. eMap and eTrex have a new feature, not available on other Garmin units, to backtrack a saved route directly. This is similar to Magellan's ability on the 315 to backtrack the main route except in Magellan's case you can't use it while collecting a tracklog since there is only one "log file".
the eTrex and eMap can backtrack any of the 10 saved logs.
You will get navigation information to the previous points and turns but,
of course, there will be no meaningful names at the turns. Since
a saved log can have up to 250 points in it, the backtrack
can very accurately reflect the turns you will expect to encounter.
(This is significantly better than the 30 route points available on other
units.) Saved logs trackback can also be reversed. You cannot
edit the saved tracklog on the unit but it can be downloaded to a computer
and edited there. You can upload tracklogs into the main tracklog
and then resave them as saved logs. While this is a bit tedious
it does permit you to create and maintain saved tracklogs of trips you
may wish to revisit.