A username is how you’re identified on TwitterAn information network made up of short messages (including photos, videos, and links) from all over the world. Sign up for an account., and is always preceded immediately by the @The @ sign is used to call out usernames in Tweets: "Hello @twitter!" People will use your @username to mention you in Tweets, send you a message or link to your profile. symbol. For instance, Twitter Support is @TwitterSupport.