Clone troopers are clones of Jango Fett (of whom Boba Fett is also a clone). It's not really known if by the time we get to the era of Luke, Han et al the Stromtroopers are all still clones or if some are and other are recruits. Perhaps the forthcoming Star Wars TV series may shed some light on this.

As I understand it (from the novels), the cloning facility (revealed to be on Kamino in AotC) is obliterated at some unspecified point in the timeline. After which, the Empire starts recruiting normal people from all over the galaxy.

However, they all have to be roughly the same build, and almost exactly the same height, to maintain appearances and the fear that comes with not knowing if the man behind the mask is one of the clones.

In fact, Han Solo was part of the Imperial military, until he defied orders and rescued Chewbacca from slavery and certain death at the hands of the military. That's why Chewie has a life-debt to Han. But while he was in the military, Han wanted to be a Stormtrooper, because that was the pinnacle of achievement for young soldiers.

...um... Like I've said before, I'm a huge Star Wars geek.

Ah, well. At least I know what makes me happy in life!