It never stops running

[epixoip]

But I still don't understand why I need a dictionnary : what if the password isn't a word, but just a mix of letters and numbers that makes no sense ?
And what does a "mask" stand for ?

"dictionary" is a misnomer, it doesn't literally mean you need a dictionary. "wordlist" is more accurate. there are lots of wordlists for download at http://www.skullsecurity.org/wiki/index.php/Passwords

if the password is random or cannot be found through other means, then you will do a mask attack. see http://hashcat.net/wiki/doku.php?id=mask_attack

there are lots of other attack modes as well, not just these two. have a look at the wiki http://hashcat.net/wiki/