The question title explains it all.
Most Helpful Girl
You can mention a user in a question's details, an opinion, an opinion comment, or an article comment by using the @ symbol in front of their username.
For example: @GirlsAskGuys
When a user is mentioned, they'll be notified (click "Mentions" from profile page). When they check that notification, they'll be taken directly to where they're mentioned on the question page.
Please keep in mind that you cannot mention a user who has blocked you, you cannot mention an invalid username, you cannot mention someone if you post anonymously, and you cannot use mentions in question titles.
This information can always be found in our FAQ as well :-)2