There was a time when Linux code was managed using the Free as in
Freedom Beer tool called Bitkeeper - a Source Code Management tool developed by Larry McVoy. But due to the differences of opinion between the free software developers and Larry, Linus decided to develop his own free (as in freedom) revision control software which he named Git. For those who are curious, Jermey had written a detailed article about the politics of Bitkeeper vis-a-vis Linux when this broke out. Now the Linux kernel source is managed using the new tool developed by Linus called Git.
Eli M. Dow has written a good tutorial which explains the basic concepts behind the usage of Git to manage source code. He walks one through the installation of Git, using Git to obtain the latest Linux kernel source tree, checking, modifying, adding and removing files to committing the changes made.
There is a still lengthier documentation at kernel.org explaining all the configuration switches of Git which also makes an interesting read.