Vsync basically limits (and synchronizes) your frame rate to your refresh rate...this eliminates image tearing (which occurs if the video card draws a second frame while the monitor's still in the process of displaying the first, so you end up with parts of two or more different frames being shown at the same time). However, a drawback of Vsync is that as a result of this synchronization (and the fact that only two buffers are available by default), if your video card can't sustain as many frames per second as your refresh rate would dictate (60, 85, whatever), the synchronization will delay the next frame until it's time for it to be displayed, so to speak...in effect, this causes frames to drop from 60->30->20 or 85->42->28 etc. (assuming your video card can't sustain 60/85/however many fps.) Triple buffering takes care of this for the most part though, but can only be forced in the drivers for OpenGL...for D3D in theory the game has to specifically request the third buffer, although I have run across a utility that seems to force triple buffering for games that don't natively support it.
My description probably is not 100% technically precise, but it should give you an idea. If you are experiencing lots of sudden framerate drops (halving), then it would be beneficial to turn Vsync off in WoW (unless it supports triple buffering, in which case you'd just enable that).