What's the difference between a strong password and a weak one?

A weak password is like "123456" or your name, which anyone can guess in two seconds. A strong one instead has a nice mix of uppercase, lowercase, numbers and special characters, is at least 12-16 characters long and isn't a word you'd find in the dictionary. Basically a weak password gets cracked in a few minutes, a strong one takes time and effort. Oh, and avoid using the same one for everything, because if they breach one site they're gonna try it everywhere.