Multiclips are an efficient way to sync two or more video clips when using Final Cut Pro. They provide an easy to switch between camera angles using a simple point-and-click or pop-up menu. Setting up a multiclip though can sometimes be painful, particularly if you want to use audio from one camera throughout while switching back-and-forth between video angles.
So when in doubt, document; I've written a brief tutorial on creating and using multiclips that's now available on the MediaStorm blog, available here.
Also, check out Steve Martin's longer essay, The Essentials of Multicam Editing in Final Cut Pro, on kenstone.net for further information.