Jimi hit the nail on the head but imma go a little more indepth..
Left Tackle- usually the tackle blocking the blind side (or the side to which the qbs back is) unless the qb is left handed.
Center - makes calls for the offencive line to block certain areas or "holes" or picks out blitzing defenders (qb's make these reads and calls as well)
Halfback/Tailback/ Running back- main ball carrier on running plays, also picks up blitzing defenders on pass plays. H-Backs are sometimes TE's an example of this would be Chris Cooley on the Washington Redskins.
Right/ left defencive end- in most cases one of them will line up over the TE (allowing them to chip block or jam him at the line on passing routes). They play the run but also utilize speed to rush the qb.
DT- mostly used to jam the line and get both the gaurds and center to block them to prevent lanes for the runningbacks to run, also frees up the de's and linebackers to make plays on running backs, or lbs to blitz the qb.
SS- used primarily to cover the te if one of the olbs is not. Also blitzes on saftey blitzes.
FS- is the centerfielder... no one is suppose to get behind him on any given play. Usually calls out the secondary's coverage audibles.
MLB- Usually the man calling the plays for the Defence, reads the offensive line coverages, and makes audibles for the Defencive line and lbs.
Nickle or dime back- Cornerback/saftey who comes in to cover the third and fourth wr's on passing downs, or anytime their is more than the normal 2 wrs. They usually line up over the inside or slot wr's.
hope that helps.
Special thanks to Nexx
I believe BK put a ton of info in there.
