Historically the switch cast predates spey casting. Basically the switch cast is exactly the same as the single-spey, but with one small difference: the fly does not kiss the water on the backcast. This small difference loses many yards on the cast, however it should also be pointed out that it also makes for a more delicate cast. So if you are casting to a fish only a short distance away then this is the cast to use.

This said, now the Switch Cast is most commonly used to refer to the Jump or Dynamic Roll. Don't you just love us flycasting instructors?