The Customised world type was one of the new features introduced by the big 1.8 Minecraft update earlier this month. It has a lot of settings you can tinker with to generate exactly the kind of map you want, provided you know what they all do. If you don't, then this video might help.
In case you're not aware, Customised worlds let you change basically everything about the map you're creating. There's easy-to-digest variables like relative number of dungeons, sea level, biome size and so on. Then there's this:
Now, the Minecraft wiki page does a fairly good job at explaining what each of these numbers do, but it's obviously better to actually see for yourself what their effects are. This is where the video comes in.
What Minecraft player pi314159265358978 did was that he took the same map (ie. the same seed), and generated it with a small change to one of the variable settings each time. He took screenshots, and created a timelapse video that, well, gets really trippy at times. Minecraft's terrain generator can do some weird things.
In HD and at full screen, it's kind of amazing.