Great advise Mcnair2Wycheck!!!
Just to add to what he said the #'s everyone keeps refering to such as 34,43,46, ect... are refering to the # of players at a specific postion.
Take the 4-3 for example. You have 4 down linemen, 3 linebackers, and 4 DB's.
The 3-4 on the other hand. You have 3 down linemen, 4 linebackers, and 4 DB's.