Quilt manages a series of patches by keeping track of the changes each of them makes. They are logically organized as a stack, and you can apply, un-apply, refresh them easily by traveling into the stack (push/pop).
Quilt is good for managing additional patches applied to a package received as a tarball or maintained in another version control system. The stacked organization is proven to be efficient for the management of very large patch sets (more than hundred patches). As matter of fact, it was designed by and for linux kernel hackers (Andrew Morton, from the -mm branch, is the original author), and its main use by the current upstream maintainer is to manage the (hundreds of) patches against the kernel made for the SUSE distribution.
This package completely integrates into the CDBS, allowing maintainers using this new paradigm for their packaging scripts to benefit from the comfort of quilt when editing their diff against upstream. The package also provides some basic support for those not using CDBS.
http://savannah.nongnu.org/projects/quilt is the current best approximation of an upstream homepage.