WordPress MU

I am trying to understand new registrations (in the admin control panel)

What is the difference between an account and a blog. Why would someone want an account but not a blog?

These are the options can anyone explain how they differ?

Disabled
Enabled for all. Blogs and user accounts can be created.
Enabled for users only. Only user account can be created.
Enabled for blogs only. Only logged in users can create new blogs.
Disable or enable registration and who or what can be registered. (Default=all)

Thank you for helping.
Helen

A user account is jus tthat - they have a username and can login. This is useful for member blogs when they have options turned on for logged in users only, like for leaving comments.

Disabled - nobody can sign up
Enabled for all. Blogs and user accounts can be created. - anyone can signup for either option.
Enabled for users only. Only user account can be created. - people can sign up for a username, but not a blog. no new blgos cna be created (except from the backend under Site Admin - Blogs)
Enabled for blogs only. Only logged in users can create new blogs. - only blogs can be created not new users. Haven;t tried this, but I gather existing users cna create new blogs, but not anyone who stumbles by, as they can;t create a username.

Thanks Andrea!