How could someone take over your webcam?

I see anecdotal stories about some creep taking over the webcam of some poor unsuspecting virtuous individual, and then viewing the hapless and unaware victim in what they think is a private setting. I’ve never heard of it happening to anyone I know, so I assume it is a rare occurrence. How could someone actually gain control over your webcam?

Story added 9. July 2013, content source with full text you can find at link above.