For a Long time we were hearing about tagging a Friend over twitter with "@" text just like @twittername then the message and the same thing was just introduced over Facebook with a tag like the "@". This tags refers a friend over the facebook to whom you wrote or about whom you wrote. These Tags are needed to link to his/her profile. You can use this tag while updating your status.
See how i used a tag to my friend over my Facebook status:
" "Friends Of the Day" is really an cool Application... My Friend @Avijit Basak Told me about this Facebook Applcication... "