3D is, if the television, film and game industries are to be believed, the future. Whether they're right or not is still up for discussion, but just in case they are, you may as well know how it all works.
Here's how the three main types of displaying 3D images in the modern era work. The top one, anaglyph, is what you will have used in the past, and may still use on select novelty products (including games). The middle one is how you will have seen Avatar at the movies, and how most upcoming 3D television sets - and Sony's 3D firmware for the PS3 - will work.
The bottom one, Parallax Barrier 3D, is the future, allowing 3D images to be projected without the need for those stupid glasses.