It's an Internet sensation in the vein of Gangnam Style, only ANYONE could do one of these videos. So, it might seem like I'm just jumping on a trend, but that's a little bit what this segment is going for, and the changes that this song debuting at #1 represents are HUGE.
So. What IS this Internet sensation? Well, first, read Sasha Frere-Jones's great article here, where he talks about what it is, what the deal with the Hot 100 is, and what some of the problems with the song are. Then, you can read the Billboard cover story about the man who made this song and sort of a history of the song. Very interesting stuff!
Then go here to see 10 fun examples of the meme.
Here's my favorite:
And here's the actual, full song itself: