If you want to add a friend's name to something you are posting, just include the "@" symbol before your friend's name. As you type the name of what you would like to reference, a drop-down menu will appear that allows you to choose from your list of friends. Soon, you'll be able to tag friends from applications as well. The "@" symbol will not be displayed in the published status update or post after you've added your tags.
Friends you tag in your status updates will receive a notification and a Wall post linking them to your post. They also will have the option to remove tags of themselves from your posts. Tagging your status updates and others posts allows you to share in a more meaningful and engaging way, and connect with even more people.
http://blog.facebook.com/blog.php?post=109765592130