WPM: Words Per Minute
CPM: Characters Per Minute
CPM is a convention used in measuring typing speed. Usually, CPM is raw characters per minute, and WPM (Words Per Minute) is derived by dividing your CPM value by the average length of the word in your test (usually 4-6).
CPM is a better indicator of raw typing speed, but is not particularly good at telling you whether or not you can handle typing actual sentences or not. Things usually get a lot trickier for touch typists when they have to handle numbers, quotations, special characters, and mixed case.
There is, of course, a more detailed write-up available on WPM.