Welcome to TB!

Get-togethers vary in their scope and structure. But most of the time, it's a number of TBers getting together to try out each other's gear, hang out and generally have fun. Sometimes, folks will bring along a drummer/guitarist/etc. and a jam will take place. Other times, a band will play a set--it all depends on the location and who's setting it up.
I've been to probably 7 or 8 GTGs in less than three years, and I highly recommend them. You get to try out all kinds of great gear in a very relaxed setting--there's no worrying about how good you are, nobody's there to judge. And TBers are generally very cool people to hang with.