If you take a look at the illustration on the right, you will see a diagram outlining the nickel defense. The Os in the diagram represent offensive players while the Xs represent the placement of the defensive players.
Notice the lowest row of Xs on the line of scrimmage (imaginary line seperating the offense and defense). You have two defensive ends (DE), one on each end of the line, and two defensive tackles (DT) in between. Behind the defensive line are two linebackers (LB).
Two cornerbacks (CB) and one nickelback (NB) line up to cover the wide receivers when the offense is in a three-receiver set. There are also two safeties. The exact position of the defensive backs depends on the type of pass coverage they are in.