As you said, the most noticeable change was when the frame went from plain to finger-grooved. The most recent change modified the extractor to better act as a loaded chamber indicator, as some (IMO, misguided) states are now requiring such things on all newly manufactured guns.
There have been several small internal modifications over the years, such as larger/differently shaped locking blocks, frame rail mods, different magazine followers and bodies, etc. None of that should impact you as an owner unless you decide to change some of the internal parts; in that case, you'd need to know what parts are involved so you don't mix-N-match a bad combination. You could head over to www.glocktalk.com and check the General Glocking forum and its subforum for more info on exactly what generation gun you have.
In any used handgun (not just a Glock), I'd make sure I fed it quality ammo (not the cheapest bargain-basement unmarked gunshow reloads); check the magazines, and if they look well-worn or the spring feels weak, get a few new ones; if it doesn't come with an instruction manual, find one and read it (I'll see if I can help with that); and for long-term storage/use, I'd suggest an extra recoil spring assembly (just a few dollars, cheap insurance in case the original breaks during shooting or disassembly/reassembly).
Welcome to The Dark Side. Most of us are pretty friendly here...
EDIT: Here's a link to an early Glock instruction manual; if you need a newer one, let me know and I'll see if I can scare one up.