Users are listed under their relevant groups. When selecting a user, you will get a few options on what you can do with their email signature as well as the preview of the signature showing on the right of the screen.
The "Display Name" is only used as an identifier for the user and not for the actual "Name" field in the email signature. For example, you could have the "Display Name" set to “GB”, and the "Name" field of the signature to “Gordan Banjac”. Most of the time though, you would have the same "Display Name" and "Name" fields.
The “Email" field is used as an identifier and is also the email address who will receive the signature when you click on the “Email to…” button.