A list of all code which can be used on the forum may be found Here
However, a short overveiw of it:

Links:
Can be created with http://google.com become http://google.com
[link]http://google.com[/link] becomes http://google.com
[link=http://google.comGoogle][/link] becomes Google
[http://google.com] becomes http://google.com

Text Formating:
[b]Text[/b] becomes text
[u]Text[/u] becomes text
[s]Text[/s] becomes text
[i]Text[/i] becomes text

[h1]text

[/h1] becomes

[h2]text

[/h2] becomes

[h3]text

[/h3] becomes

[size=1]text[/size] becomes text
(you can replace the number with any number, even negatives)

[color=red]this is red[/color] becomes this is red
[color=#ff00aa]this is a hex color[/color] becomes this is a hex color

[sub]Text[/sub] becomes text
[super]Text[/super] becomes text

[hr] becomes

---

[list]

[-]item 1

[-]item 2

[-]item 3

[/list]becomes

• item 1

• item 2

• item 3

[code]this is code

[/code]becomes

this is code

[spoiler=Some Game]this is a spoiler.

[/spoiler]becomes

[quote=Some Person]this is a quote

[/quote]becomes