Easily observed differences:
-1st generation smooth grip no finger grooves no rails
-2nd generation textured grip no finger grooves no rails
-3rd generation textured grip finger grooves accessory rails. Loaded chamber indicator incorporated into the extractor.
Not so easily observed:
The 1st and some of the 2nd generation 9mm Glocks were built on two-pin frames. All are now built on three-pin frames. There isn't a performance difference in 9mm. Though, you just can't put a .40 upper on a two-pin frame because it'll wear it out quickly.
In mid 1995 Glock changed the breech face and extractor from 90 degrees back to its original 15 degress.
There were also trigger part upgrades to the earliest models (these have black metal trigger parts and a black metal serial marker on the lower). If you have an early Glock with the black parts, you can get a free upgrade from Glock. I've never seen them -- 99.99% have been upgraded.
There are also a host of magazine design changes (though all mags work in the models for which they were designed, regardless of generation).
Overall, the 9mms will be the most reliable and durable of all the generations and there should not be any problems with them. The .45acp, .40S&W/.357sig, and 10mm Glocks are not as reliable and durable as the 9mm models, but they are better in 3rd generation(not as good as 9mm though).
If you're really interested in this sort of thing, Pat Sweeney's Gun Digest Book of the Glock is a great source.
