If you take a look at the illustration on the right, you will see a diagram outlining the Over 4-3 alignment. The Os in the diagram represent offensive players while the Xs represent the placement of the defensive players.
Notice the lowest row of Xs positioned on the line of scrimmage (imaginary line seperating the offense and defense). The two defensive ends (DE) line up over the offensive tackles. The two defensive tackles (DT) line up over the center and the guard on the strong side. One linebacker (LB) lines up on the line of scrimmage over the tight end. Another lines up off the line, just outside the weak-side defensive end. The third linebacker lines up off the line, but in the gap between the weak-side tackle and end.
When in the Over 4-3, the strength of the defense is on the offense's strong side. With a simple shift to the Under 4-3, the strength of the defense moves to the weak side. To get from the Over formation to the Under, simply shift the defensive tackles over one spot so that they are now over the center and the weak-side guard. The linebacker playing the gap between the weak-side tackle and end then shifts to the gap created between the strong-side tackle and end.
Two cornerbacks (CB), one on each side of the field, line up to cover the wide receivers. There are also two safeties. The exact position of the defensive backs (cornerbacks and safeties) depends on the type of pass coverage they are in.