CANNOT/CAN NOT. These two spellings are largely interchangeable, but by far the most ... when you want to be emphatic: "No, you can not wash the dog in the Maytag. ... http://www.wsu.edu/~brians/errors/cannot.html
"Can" is a word that means is able to, knows how to, is potentially capable of. "Cannot" means is not able to, does not know how to, is not potentially capable of. I don't see why one would need to use "can not" but it seems that many people ignore "cannot" altogether. Probably the same people who ignore the existence of the word "may" in favour of "can".