Comment 11 for bug 41096

"ubuntu" in the version means that they're modified, compared to the Debian versions. Sometimes, this is a small bugfix or tweak, sometimes it's us shipping a new upstream before Debian, and sometimes it means we're pretty painfully forked (as is currently the case with Thunderbird, but that will be resolved after dapper releases).

If you're curious as to how different a package is from Debian, "zless /usr/share/doc/<package>/changelog.Debian.gz" is a good place to look (search for occurances of the string "ubuntu" to see each ubuntu revision, we splice our change entries in where appropriate)