Composite video, also called baseband video, usually comes out of a single (sometimes yellow) RCA style jack. It's the original video signal format.
A composite video splitter takes a feed from a single video source, such as a camera or cable box, and provides outputs that can go to video input jacks on multiple devices. There's not much to a composite video splitter; it just need to electrically "buffer" the video signal so that the signal level doesn't drop too much. While you can split a composite video signal with a simple "Y" splitter cable, the resulting picture would be noticeably darker on the destination devices because the cable does not buffer the signal.