TCR files are an extension file for the eReader that you may be using right now. They were originally files that were part of the core of eReaders, but they have since moved to being used more specifically as an extension. These files are largely on their way out it seems as other types of files have mostly taken their place in the eBook world. We are lucky to have had TCR files as an early stepping stone in the process of getting eReaders out to the world, but the TCR files of today just don't have the same pull that they once did. It is important to recognize that many file types are used for a while before ultimately becoming discarded in favor of other options.