VATSIM, or Virtual Air Traffic Flight Simulation Network, is a virtual "World" which provides flight simmers to be provided with air traffic control from [real] Air traffic controllers around the world.
To use Vatsim, first you must join, then get SquawkBox3 or FSINN [i use SB3]
If you are considering joining, please read the
Pilot Resource Centreand
Tips for New PilotsWhen you are connected, monitor the traffic in your airport and get with the lingo of simmers

Hope this helps

Cheers,