had a server get cracked like this once
setup a dev server so our filipino outsource guys could login, and they had trouble typing my old root password ('sxt7,!#rsQQbfp!') so i let them change it to "whatever you want". they chose "p455w0rd", and i shrugged it off.
6 hours later, i got an email from amazon saying my server had been comprimised and was making a ton of requests to SSH into other servers.
i logged in to take a look, and found a big passwords.txt full of 20k common combinations of common shit, and a tiny script that continually tried passwords on random IPs until it found one that works. grepped the passwords.txt to find my password, and voila, it was #8 on the list to try. *sighs*
It's called bruteforcing