An Irish and Welsh form of Catherine, Caitlin was a boom name of the eighties, rocketing from obscurity (Americans first heard it via the wife of doomed poet Dylan Thomas) to the height of popularity in the space of a decade. The original name was gradually eclipsed by its myriad spelling variations--Katelyn and Kaitlyn both soon topping it on the popularity lists.
Now, most forms of Caitlin, including this most-authentic one, have started to dip, though there have been thousands of girls named Caitlin -- and Katelyn and Kaitlyn ad infinitum -- every year for the past three decades. So, while Caitlin is certainly one of the loveliest variations of Catherine, it's been overused to the point of semi-retirement, at least in the U.S. Cait might be a short form.
Actor Breckin Meyer named his daughter Caitlin in 2003.