Most wireless and PC sync adapters will only allow you to fire a manually set flash, without any TTL metering or camera-based control of the flash's output.
An off-camera-shoe-cord allows you to use full eTTL control of the flash. An ST-E2 or the camera models with built-in wireless control do, too, wirelessly and with the ability to control multiple flashes. But an ST-E2 (or the camera models with built-in wireless flash control) only work line-of-sight because they rely upon IR for communication. Why is this important? Well, line-of-sight significantly limits where you can position the flash and makes it more difficult to avoid hot spots or accidentally getting the flash in your image.
There are a few radio wireless with some sort of eTTL control starting to appear on the market (I think Paul Buff makes one, but suspect they're still pretty pricey). To do this, these have to translate the signal to radio, which doesn't need to be line-of-sight, then back to IR.