3-4 Defensive Formation
The 3-4 defensive formation is the other more popular formation for a defense in football. Again it is named for the combination of linemen (3)and linebackers (4). So, to state the obvious, this defense swaps a lineman for an extra linebacker on the field.
If this defense was lined up against an offensive formation it would look something like this.
There is a single nose tackle who lines up over the center of the offensive line and there are two defensive ends.
Since there are only 3 linemen, they are generally a bigger guys than the linemen of a 4-3 defense. Make sense? There are less of them to try and do a lot of the same stuff against the same 5 guys of the offensive line. Of course, they will get help from the linebackers too but the lineman are the first line of defense.