An account is NOT a username. Your account is set up on your email address. You can have 1 profile on a free account and up to 12 profiles on a supporter account. To have a username, you need to have at least one profile on your account.
If you have more than one profile, each profile will have a separate inbox and separate image folders. You can switch profiles without logging out in order to read messages or access albums for each profile.
Image storage is based on your account id, e.g. 12 profiles on an account does not increase the amount of storage you have available on that account.
Each profile will have its own profile page and signature(s).
Each profile manages its own friends.
You can add a blog to each profile page.