If you take a look at the illustration on the right, you will see a diagram outlining the 3-4 Eagle 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 near the line of scrimmage (imaginary line seperating the offense and defense). In this formation, you have just two defensive linemen on the field, normally defensive tackles (DT). In the middle of the line, where the nose tackle would normally be, is a linebacker (LB). Two more linebackers line up as ends, outside the defensive tackles. The last two linebackers line up behind the defensive line.
Two cornerbacks (CB), one on each side of the field, line up to cover the wide receivers. There are also two safeties. The exact positioning of the defensive backs (cornerbacks and safeties) depends on the type of pass coverage they are in.