There are essentially three, maybe even more that I don't know about.
Freshmen typically have their own league. Sophomores and some juniors will typically fill out a junior varsity team if they're not receiving substantial playing time with the varsity as well (you can tack freshmen on as well). There is also, of course, the varsity.
It varies, though.
In Harrisburg, Arkansas (at least in 1992), freshmen played with the junior high team, and that's generally how it worked for the other schools in the area as well. Sophomores and up all played varsity. We technically had 5 "B" games scheduled for the backups, but they were never played. We really didn't have the depth to have a "B" squad, anyhow.