Make certain that you download the CDisplayEx or CDisplay programs (such as http://sourceforge.net/projects/cdisplayex/) to open the .cbr files so that you can easily read the comic books that you've downloaded. I haven't tried Comic Rack software myself, but it is available here: http://comicrack.cyolito.com/downloads. When using .cbr software, please do not forget that once the comic book reader opens the file to right-click on the comic book cover and then click on "Options" and then click on "Fit to Screen" so that it enlarges everything so that it is readable. If you do not do this you will surely STRAIN YOUR EYES!
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.
Dancing Tortoise Software: http://dancingtortoise.com/simplecomic/
You can find a more in-depth list of readers on Wikipedia:
I hope this is helpful. If you have any questions, or additional information that you feel should be included here, please post below.