Simply put, they are what are know as Comic Book Archive files, something that has become a sort of a de facto standard when it comes to putting out scanned comic books onto the web. Basically all a .cbr or .cbz file is, is a stardard ZIP or RAR archive file (the ‘z’ or ‘r’ at the back gives away which one) containing a number of image files.
These file types are automatically opened by so called comic or manga viewers, and are thus a very useful way for keeping comic books consisting of multiple scanned images in a rather organised fashion.
The Wikipedia entry on the matter lists some useful comic book readers in case you are interested in delving deeper into this little world.
And now you know.
Related Link: http://en.wikipedia.org/wiki/Comic_Book_Archive_file