Quoted from Wikipedia: "Comic Book Archive file or ComicBook Reader File is a ZIP, RAR, or more rarely TAR (tar, tar.bz2 and tar.gz) file of images with the file extension renamed to .cbz, .cbr and .cbt respectively for the purpose of viewing the images in sequence with an image viewer and simplifying file maintenance."
Basically, .cbr and .cbz files are simply RAR or ZIP files that have been renamed. If you want, you can rename them to .rar or .zip and extract the .jpg files inside.
How do I read them?
There are several popular .cbr and .cbz readers. My personal favorite is CDisplayEx.
Simple Comic http://dancingtortoise.com/simplecomic/
You can find a more in-depth list of readers on Wikipedia: