Usernames with one @, such as "@fawn", versus usernames with two @s, such as "@firstname.lastname@example.org", indicate whether or not the user is on the same instance as you. Users that are on the same instance as you will only have their username displayed. Users that are on a different instances than you will have their username and instance displayed in an email-like format. The indication of the instance the user is on is especially important when mentioning that user in a post, or trying to search for them; to mention a user in the same instance as you, you only need their username (@fawn), while you will need to use the entire username and instance format (@email@example.com) when mentioning users outside of your instance. The same is true when searching for users.
Back to FAQ.