Rather complicated skateboarding term. Switch is used in several contexts, it basically means riding the opposite (or the switch) way to which you normally ride.

ex. If you are regular your left foot is your lead foot. Therefore if you switch to your right foot leading you are riding switch. Viceversa for goofy footers.

In some cases it is not called switch, but fakie. This is how and when the terms are used:
1. If you are riding in your normal stance and you do a 180/540/900 you come out riding switch.
2. If you begin a run (by pushing off) in the opposite stance to your regular stance you are riding fakie.
3. If you are riding switch or fakie and you grind you are doing a switch grind.
4. If you are riding switch or fakie and you go off a jump you are doing a switch trick.
5. If you are riding switch or fakie and you do a 360/720/1080 you come out riding switch.
6. If you begin grinding up a rail in your normal stance - for this example doing a nosegrind - and start coming back down you are doing a fakie 5'0.

Fakie is usually worth more points than switch in a competition, though fakie is a far less common move.