The Defensive Side of the game:
Defensive Tackle: These are the big men in the defensive line who are expected to clog the middle of the line of scrimmage to stop the running backs running into these gaps. Expected to pressure the quarterback to fore him out the pocket as well. ( Ndamakong Suh, Vince Wilfork, Kyle Williams, Geno Atkins)
Defensive Ends: These guys are either side of the Defensive tackle and tasked with stopping the run or trying to sack or hurry the Quarterback. ( JJ Watt, Haloti Ngata, Justin Smith)
Middle Linebacker: They cover middle of the field as the name suggests, stopping the run and covering the middle of the field is what they might be expected to do. ( Patrick Willis, Ray Lewis Brian Urlacher)
Outside Linebacker: The Outside linebackers are to the left and right of the MLB, some of them are used just purely to rush the quarterback and pile on the sacks whilst others help with stuffing the run and coverage. ( Jerod Mayo, Clay Matthews, Demarcus Ware, Von Miller)
Cornerbacks: These guys are supposed to stop the Wide Receivers and sometimes Tight ends they have an extremely hard job since the game has moved to be favorable for the offensive side of things. (Darelle Revis, Richard Sherman, Asante Samuels)
Free Safety: These guys are the last line of defense on the field, they are used as a safety blanket for Cornerbacks and try keep everything in front of them. They cover the weak side of the field. ( Ed Reed, Devin McCourty, Earl Thomas , Eric Weddle)
Strong Safety: These guys are usually smaller faster "linebackers", help with run support and covers the strong side of the field where the tight end lines up. He plays closer to the line of scrimmage then the Free safety. ( Troy Palomalou, Kam chancellor, Adrian Wilson)
Defenses play usually either man to man or zone defense. Man to man as the name suggest is you each have a man who your job is to cover whilst the Safeties usually double up on the best offensive players to help out. So you might hear the term double teamed.
Zone is each player has a zone their designated to cover, its much more complicated i think and prone to being gutted by big plays but more forces more turnovers.
Another version of Man to man is Press coverage in which the Cornerbacks try and get physical with the wide receivers and stop them from getting past them by blocking NOT Grabbing them. Usually done by big physical corners.
All three have benefits and negatives, as well as many different smaller sub plays.