Kitten is the trimmed down DOS specific version of Cats.
Cats is Jim Hall's implementatin of the (often found on Unix/Posix systems) catgets message library.
But what is it?
It is a library (collection of software routines) that allows a program to easily support multiple languages. The primary language (usually English) is still compiled in, but at run time a file containing translations for the user's selected language is loaded and replaces the compiled in messages with ones in the given language. The message files are simple text files, which can easily be translated by anyone who knows both languages, and then provided to other users.
doshea has developed a Perl script called MUSCHI which allows translation to be done at compile time while still allowing run-time selection of the language as per Kitten's normal functionality. It will be included and used for the next release of Mem.
The version of kitten which adds the least size to a program is the kitten from FIND. The version of kitten included with MD5SUM is based off of this version and can be compiled with Turbo C, Watcom C, Pacific C, and Micro-C. The original version of kitten is likely the most TSR-friendly version, due to its low memory consumption, whereas the kitten from FIND consumes the most memory, due to using fixed strings.