Things to remember:

With a 4-3 base defense, there are four linemen and 3 linebackers. That leave 4 defensive backs (2 corners and 2 safeties). This defense is great against the run and 2 wide receiver sets.

With a nickel defense, you substitute one of the linebackers (bigger and slower) for a 5th defensive back (smaller and faster). So you have four linemen, two linebackers and 5 DBs. This defense is usually played when there are three receivers on the field. It is better against the pass than the run.

With a dime defense, you substitute one more of the linebackers for a 6th defensive back. This defense is designed to stop the pass and is pretty bad against the run because there is only one linebacker. The line is thinner with 4 linemen and one linebacker.