Output Buffer
If you want to read a great article, check out this article on php output buffering at phpit.
All you really need to know is that Skeleton doesn't print anything to the user until the script is done processing. Why is this useful? Because you can modify header and cookie information at any point in Skeleton without getting any "Cannot modify header information - headers already sent" error. Cool huh?
Oh yeah, it also lets us put our header, footer and navigation in one file instead of having to have separate files for each.