VST's are plug-ins like reverbs, compressors, amp simulators, echoes.. just about anything you can think of.
VSTi's are also plugins but the 'i' stands for instruments. So it's stuff like synths, organs, samplers.
I've got a bunch of pics of stuff in this tutorial:

http://www.guitartricks.com/lesson.php?input=7452&s_id=58

You won't be able to hear the examples unless you register but the pics and description should give you a decent idea.