I was one of nearly 4,000 people participating in Eric Whitacre's Virtual Choir this year. We're not allowed to embed, but you can see it on YouTube here:
The basic principle is simple: if you get a bunch of people to take video and audio if themselves singing to the same song, you can stitch it together in post-production and create a virtual choir. Anyone who does multitracks (Corey Vidal, FineyLeee, Peter Hollens) uses the same tactics.
What makes it special is Eric Whitacre. For those not following this stuff (and let's face it, no one is), he's one of the two star composers of the world of choral music right now, the other being Morten Lauridsen. Whitacre came out of nowhere in the early 90s and started making a name for himself, choosing exquisite poems and then crafting around them music of surpassing beauty, music that melts in your mouth without being impossibly high-brow. The piece he chose for this year's virtual choir, Water Night, is probably his signature work. It is spellbinding. Most choral pieces are written with a total of four notes at once, corresponding to the four basic voice parts (soprano: women who sing high; alto: women who sing low; tenor & bass: men who sing high / low). Water Night goes up to fourteen.