WHAT ARE THE A GAPS AND B GAPS?

[HOOP]

I have my 2 young cousins coming over today to play some EF. They both play pop warner football so they will have some of idea of this when i explain it to them. but i want to make sure i explain it to them right. so to clarify what i think i already know, can someone explain

the gaps and the responsibilities of the defensive player in the different schemes?
also, the 5 technique and so on....?

thanks fellas for the help,

scott