'Tagging' another user is a useful way to draw someone's attention to a particular thread. Likewise, 'mentioning' another user draws their attention to a specific post, similar to the @comment used by Twitter. When either tagging or mentioning a person, the user will receive a notification in the upper right, and may receive an email, depending on the settings.
To tag a person, simply click the "Tag User(s)" button at the top left of the page and start typing in their username
One way to "mention" another user is to add the @ sign before and ; after the username. Example: @BigAl205;
Another option is to click the 'Mention' button on the editor and put the username between the tags
Note: The 'mention' system only works with simple text, such as typing a person's username, or copy and pasting from a simple text format. Copy/pasting a person's username from their profile doesn't seem to work properly, since you would be copying a link instead of simple text.