In essence, the Common Debian Build System attempts to provide a modular yet very powerful build system for Debian packages. You can use it to create a build system for typical GNU autotools packages in 4 lines, or you can just reuse the CDBS core to implement your own build systems.
Besides autotools.mk, there are also a number of useful other premade classes, such as ones for Perl and Python. Since implementing classes is so easy, you can write your own class for a collection of similar packages, and then propose it for inclusion in CDBS when it's ready.
See this document.
CDBS itself comes with an introduction, as well as a number of examples.
See the CDBS project page.
