4-3 Defensive Formation
This is one of the more popular defensive formations along with the 3-4. It gets its name from the fact that there are 4 defensive linemen and 3 linebackers on the field. That leaves 4 more players which generally consist of 2 cornerbacks and 2 safeties.
The Defensive Tackles would line up over the offensive guards.
The defensive ends line up to the outside of the tackles.
The middle linebacker would essentially line up in front of the center.
Everybody has a zone that they are responsible for watching offensive players coming into but that can change play to play depending on blitzes, the offensive formation, etc.