A comic book reader software does not use a special format of images; the extension .cbr is given by the type of archive. The other archive types used are .cbz, .cb7, .cbt and .cba. This type of programs offers some options for the user that makes the act of reading a comic book on an electronic support, more pleasant and realistic. Some of them have a click and drag option that lets you move the page, with the help of the mouse, thing that makes the user feel the entire experience of reading a comic book. The other options are the classic ones, like page forwards and backwards, print or zoom.
The Marvel comic site is currently having a comic book reader incorporated on their platform. If you pay the fee that the site implied, you have unlimited commix for a month. But you can also find comic book reader software available for download.
If you are a Linux user you can use Okular, a comic book reader that includes PDF format. Other good programs are Comix and Evince.
For Mac users are available Simple Comic and Comic Zeal. Comic Zeal is an app that can be found on the Apple Store.
And finally, if you use Windows, you will have more options. Evince is also available for Windows and besides this one, there are also Sumatra, STDU Viewer, CDisplay and Calibre.
Regardless of what comic book reader you will choose or how old are you, reading a comic book is always a fun way to kill your time, but it can also be a hobby.