BitKeeper is the VersionControlSystem that was used for KernelDevelopment since sometime around 2.4.13 until 2.6.12-rc2 when it was replaced by Git due to a fallout between the owner of BitKeeper and OSDL. Its major distinguishing characteristic is that it keeps distributed repositories, unlike the CVS model where everyone commits to a central repository located on a single host.
It is not FreeSoftware, and does some strange things like making all your changes/log files public unless you buy a commercial licence. (The idea is that OpenSource projects can use it without charge, but people who want to keep changes proprietary need to pay up.) It seems to also require you to accept any changes to the licence whenever BitMover choose to make them, and to stay upgraded. Another contentious issue is that they changed their licence so that people working on a competing product, eg SubVersion, cannot use it. There has been a lot of anger over this move, probably rightly.