how do grease traps work?

Answer 1

This link will give you an illustrated answer to your question.
http://www.oracwa.org/Pages/trapframe.htm

Answer 2

It traps the grease from drain lines so they wont clog. But you have to get the grease trap regularly cleaned too. The solid goes to the bottom of the trap and the grease floats on the top. In the middle is just regular water. The water flows through the trap (as this is what you are draining off, just like when you wash your dishes and the water drains down the pipe, same thing, but a grease trap will stop the grease from clogging your lines). It has to be cleaned often to keep it working. I manage food service and we have to get ours done every 3 months. Some places can wait and do every 6 months, depending on how much oil and food ends up in the line.
The person above me who answered gave a link that shows a pic, which can also show you how it looks inside.