They are a great addition to flight simming especially after you run out of places to go and things to do. Gives a little more direction.
You basically pick your interest. Some are very formal and require you to pass tests to join and others are informal letting you do your own thing as much as possible.
You also have the choice of what kind of flying you like best. If you ever dreamed of being the captain of an Airbus or 747 and making long haul flights. There are lots to choose from.
If you love to go low and slow in back woods areas there's a couple bush flying organizations also. The one I know best is BFU (Bush Flying Unlimited).
http://bfu.avsim.net/ It's very informal. You pick the job you want from a message board. Post a new one for someone else and fly the job (online of offline). When you are done you write up a PIREP and paste that in the thread where you picked up the job along with some screen shots.