Q: What's the difference between display names and usernames?
Flexible usernames (display names) are shown at the top of your moblog and by comments, while usernames are purely for logging in to the site and posting images. Your username cannot contain spaces or punctuation, but your display name can, and it can be longer as well.
You cannot change your username, but your display name can be easily changed anytime you feel like it. By default your display name is set to your username.