New to buying lenses

[eye-capture]

So far, everything I have had been passed down to me. Anywhoo, what do I need to know about buying new lenses?

I'm pretty sure "IS" stands for Image Stabilzers
But what about that USM stuff?

Can someone explain to me what I am looking for and what I shuold be aiming at?

Edit (thanks to usayit):

My budget is very low, I'm down for about 200-300 per lens right now. I'm shooting with a Pentax IST DL which requires a K-Mount lens. I was thinking of the Sigma 24-124, not quite sure the specifics on it, but some people weren't a big fan ofr that lens.

 

[usayit]

USM... UltraSonic Motor

Faster, quieter, and more accurate motor used to focus the lens elements pioneered by Canon. This link should help with the various technologies Canon offers:

http://www.canon.com/camera-museum/tech/room/f_index.html

First... people here will need more specifics on your question. Budget, what do you need (shoot), camera mount ( canon I'm assuming) etc...