I've been reading about Cisco IOS releases from this site: http://www.cisco.com/warp/public/620/1.html
and it's so confusing! It there a simple way to understand what all the T, S, etc. means?
It used to be even harder!
Check out Stretch's cheat sheet over on packetlife...
Regular IOS routers would use the 12.4M (Mainline) or 12.4T (Technology). You will find the T train to contain the newest features but it may also contain the most bugs. We often recommend Mainline over T train if you are interested on stability in your network without the need of new features.
Other tracks would be platform specific, for instance;
12.2SX = Catalyst 6500
12.2SE = Catalyst 3xxx switches
12.2SR = Cisco 7600 Routers
12.2SB = Cisco 7200 and 7300 Routers.
There are a lot of tracks out there and I can understand your confusion but if you stick to the main ones, you should be ok.
Any track in particular you are looking for?
You can also refer to this for more info
In general, the various IOS releases
T = Testing and training releases
ED = Early deployment
GD = General deployment
Normally T releases are the versions that were released for ad hoc bug fixes
ED are initial stages of deployment but still subjected to bugs.
GD are the most stable versions and can run for a long time before the next maintenance reboot.