Ever look at the filename of the Cisco IOS image on a router and wonder what all the letters mean and what capabilities are included in your OS?
To find out, you can go through a few dozen pages in the IOS Reference Guide, or the much longer Cisco IOS Packaging (Product Bulletin No. 2160).
If you want a quicker way to find out what's in your image, you can use the Cisco Feature Navigator. It's really simple. You put in the image name that your device boots from and it gives you a list of what features you've got. Figuring out those individual features is left as an exercise for the reader ;-)