Well, CAT means "computer-assisted translation", so this means that it is not completely made by the computer, it just "assists". CAT software helps the translator do his job faster by saving what he/she has translated so far. Thus, every time he/she runs across a segment that he has already translated (or that he has translated in some degree at least), he'll be offered his own previous translation. CAT tools are not a substitute for a translator, nor automatic translation software such as Google translator is. CAT software only offers you segments you already have in a Translation Memory.
I hope this helps you a little bit.
See more here:
http://en.wikipedia.org/wiki/Compute...ed_translation