What is a (LETS) Local Exchange Trading System?

The most difficult part about starting a LETS is probably the challenge of trying to explain to people exactly what a LETS is.
What it does? How does it work? Why should i worry about joining?

The following video is a good intro to the system.

https://www.youtube.com/watch?v=koK2wUwj2l8&list=LL&index=3