If you know how to use HTML, you can use an abbreviated list of HTML tags in your posts, including a, b, blockquote, center, em, i, li, ol, pre, small, strike, strong, sub, sup, and ul. You can just type them into the compose/comment box.
To add italics, for example, you would type: <em>italicized word</em>
which shows as: italicized word
To add a link to example.com you would type: <a href="http://example.com">here is a link to example.com</a>
which shows as: here is a link to example.com
If you use a modern browser, you'll see B/I/link buttons below the compose/comment box that you can use to add basic formatting. To make a link using the button, you highlight the word or phrase you want to be the link text (like the phrase "here is a link to example.com" above), click the link button, and type or paste the URL into the box that appears.
Here's a short screencast showing how to use the buttons to insert HTML for you: