When you are composing a new post on a forum, you have two different editors to choose from to create the post with. The Preview Editor allows you to write your posts and add styles to it like you would when using a traditional word processor. This allows you to type out a message and add any styles to your post that you would like, and you will see all of the styles that you have selected appear in the message area so you can have a preview of what to expect when your post is made. The other editor that's available is called the BBCode Editor, which is a traditional message editor that's used in older forum software. The BBCode Editor allows members that are familiar with BBCode to stylize their posts using BBCode tags rather than getting a real preview representation of the style that you are attempting to add to your post.
You have the option to choose which editor you want to use as your default editor when composing new posts. To do this, click the Profile button found in the top menu bar, and then click the Edit Profile button when on your profile page. You'll then be taken to the Edit Profile & Settings page, once here, click the Settings tab on the page. Next, look for the Posting Default option found about half-way down the page, simply click the Preview button or BBCode button depending on which editor you would like to use.
If you click the BBCode button, you'll notice a new option display for Default BBCode with a text box below it. Anything you type in this box will always appear when you load the posting page when using the BBCode mode. If you are someone that always wants your posts to appear in a specific color, or in a certain style, you can set those styles in this box here so that they will always be available to you when you go to create a new post, rather than having to manually set your post style each time you create a post.