There's a lot of folklore in the Debian community about how apt works. Unfortunately, a lot of it is also just plain wrong. For instance, I often hear people confidently say things like aptitude is a frontend to apt-get (it isn't).
In the interest of clearing up some of this confusion, I've created some simple diagrams of what uses what in the apt system. apt is large and complicated, so these diagrams can't be complete, but I hope they at least clear up the basic relationships between the different pieces of software.