I'll be heading to Lime Rock next weekend myself as well
Check out the ALMS website for the rules, but the biggest difference between ALMS and most other racing is that there are 4 different classes of cars running on the track at the same time. So you will see the Audi TDIs blowing by the M3s and Acura prototypes whooping up on Corvettes....it's pretty awesome.
It's a great time at Lime Rock and remember, the coolest thing about ALMS is that before the race starts, they let the entire crowd down to the track to check out the cars. It's part of the charm of ALMS.