Guards are the lineman that do a lot of the dirty work for an offense. They are especially important in the run game. They must be athletic enough to pull and trap, while also being able to hold their ground and drive back big defensive tackles. Guards have to be able to recognize inside blitzes and stunts and help out the center if needed.
A prototypical NFL guard is at least 6'4'', has excellent feet, long arms, great strength, good hand placement, keeps low, takes good angles to get to their man and can move. There are three or four guards in this year's draft that could potentially start right away in the NFL. It's a solid group, but there is quite a drop off in talent and potential after the top-end talent.
Here are my Top 10 Guards of the 2011 NFL Draft.