Defensive Schemes

Question
What do 3-4, 4-3, Nickel, and Dime schemes mean?
Are some primarily run defenses and others pass?
What criteria is used in determining which scheme to use in a given situation?

Thanks a lot!

Answer
Dear Tim,
 The first number in the 3-4 and 4-3 means how many linemen there are.  The second number is for how many linebackers.  In the nickel there is five defensive backs and in a dime you usually get six.
 You use these depending on what you are going up against.  If a team is throwing the ball all over you, you'll want extra DB's in there so you may play a dime or nickel, if they are running well, then you will probably use the 4-3 or 3-4.

Good luck,
Chris