Hej Fristi,
since you asked, I don't want to leave your question unanswered:
The Eszett ("ß") is used after diphthongs and after long vowels, two "s" after short vowels.
In any case, there is always only one correct form, using the other is a spelling mistake.
This is complicated by a number of things: In 1996 there was a huge reform for the orthography of the written German language. It was an effort to make things less complicated. In reality, some things were improved, some were changed in bad ways, some things that should have changed stayed the same, and in the end it was in some aspects more confusing than before (it was even suggested the the main point of it was to sell a huge amount of dictionaries and other books that had to be reprinted). The Eszett is one of the things that were at first supposed to get abolished in this reform, but stayed anyway - but the rules for it's usage changed, and with it many words. Many people, including native speakers, are still insecure about the correct usage and make mistakes.
The other problem is that there is no capital version for the Eszett, so when you write in all capital letters, the Eszett is replaced by "SS". This can cause further confusion, as some people will read a word like this and assume it is correct with "ss", not realizing that this is only because of the capital letters.
And finally, there are people who refuse to use their own language correctly when writing and just always write "ss", thinking that this is an acceptable solution. A poor choice, as I see it, but time will tell; languages are always evolving.
If you want to know more about this strange little letter, check it out on wikipedia:
http://en.wikipedia.org/wiki/ß
Lord Nimrod
