When you write things to appear on the site, be it an article, a comment, a forum post, or a gallery description, you type into a box on the screen. It's all quite simple, but all your text looks the same.
There are in fact several formatting tools available, and to see them you must click the small arrow at the right of the top border of the box. This expands the border and shows an array of buttons, similar to a word processor.
If it looks scary, then don't bother with it, but if you're used to word processing the buttons will look familiar. The middle row is the most useful, with buttons for things like bold, italic, etc., bullet lists, and left/right justification. Use them to help make your text more readable.