All forum posts are sanitized, meaning all HTML is escaped. That being said, there are a few features you can use:
Any URL starting with http:// will automatically be converted into a link.
Description: Link to a specific post number, where 12345 would be the id number of the post.
Result: Post #12345
Description: Link to a forum number, where 12345 would be the id number of the topic.
Result: Forum #12345
BBCode: [b]Bold Text[/b]
Description: Bold a part of your message.
Result: Bold Text
BBCode: [i]Italicized Text[/i]
Description: Italicize a part of your message.
Result: Italicized Text
BBCode: [s]Struck-out Text[/s]
Description: Stike-through a part of your message.
BBCode: [spoiler]The spoiler is a lie.[/spoiler]
Description: Hide text that can be shown by clicking it.
Result: The spoiler is a lie.
BBCode: [quote]Someone said: Some stuff about cats[/quote]
Description: Quote part of all of what someone said.