For the most part, your posts will contain plain text, but on occasions, you may want to emphasize certain words or phrases by making them (for example) bold or italic.

Depending on the rules of the forum, you may use HTML code to produce these effects. However, more often than not, the administrator will have disabled HTML code, and opted instead to use a special set of tags which you can use to produce the most popular text-effects. This has the advantage that it is very simple to use, and is immune to malicious javascripts and page layout disruption.