These patches are generated daily and contain the differences between an Ubuntu source package and the equivalent version of the same source in Debian. This means that the base of the patch for an Ubuntu 1.2-3ubuntu4 version will be the Debian 1.2-3 package, even if the Debian version is now 1.4-1. This hopefully makes the packages easier to merge.
Where Ubuntu have packaged a new upstream themselves, noted by a revision beginning 0ubuntu the patch is from the first Debian revision of the same upstream version if available (so 1.2-0ubuntu1 will be compared from the Debian 1.2-1 package). Where not available, you will usually find that the base of the patch is the first common ancestor of both Ubuntu and Debian.
The layout is the same as a Debian FTP pool, so to see the changes made to debhelper see the d/debhelper directory and to see the changes made to libraw1394 see the libr/libraw1394 directory. The filename within that directory indicates the current version of the source in Ubuntu.
A machine-parseable index of the patch set is available in the PATCHES file. The format of this file is one source package per line, with the name of the source package and relative path to the current patch file separated by a space on each line.