We don't need to give you a lengthy writeup here. The YouTube video description will do quite nicely: "Every time* anyone says the words "crazy" or "baby" in any of [Britney Spears'] songs. In chronological order." It's like an Evolution of Dance-Pop video, or maybe an Evolution of Britney Intonation video, except it's a lot more like a skipping record!
*Except for the part where one guy sings "baby, baby, baby" for 1:15 of "Get Naked (I Got a Plan)," which is sped up. Thanks, person with lots of spare time and ideas!