Specialty Equipment Market Association.   All the aftermarket stuff for stuff with engines for the most part.   The booths are people trying to get their products out to shop owners.   Anything from lifts/alignment/ac machines and all other equipment in one place so you can see the latest and greatest in action instead of a salesman coming by with a pamphlet.   In another hall there's all the wheels and tires and whores with no clothes on leaving slug trails on 30" wheels.   There are offroad based areas, lift companies etc. 

The large companies have a big showoff booth, everyone knows what they have for the most part, they are showing off with a huge expensive display and crazy vehicles.   The smaller 12x12 booths is where you can see the stuff you've never heard of and see what is out there in the market.   The paint and body area has an incredible amount of neat paint jobs.  

I don't go to car shows for the most part, so SEMA was cool for me.  Lots of neat ****, and we did hook up with a chinese LED supplier and start bringing crates of lights in (2012).   Haven't been back since.