Panel login: logs you into your DreamHost panel, where you manage your payments, domains, Users, etc.
-You can create additional logins for your panel account. these additional accounts can be given limited options (for example, you can limit a new account to only managing certain domains).
Users: you manage users in your DreamHost panel. you can have one or more users. Each user account will handle one or more websites. When you set up a domain, it will ask what user you want to put the website under.
users, are quite literally, user accounts on a computer. DreamHost is giving you a user account on a computer somewhere. You can use SSH to login to a computer with that user-account.
Domains and websites...when you set up a domain, you tell Dreamhost you want that domain to point to a folder under a specific user account. Then a folder is created and that domain will point to that folder..which is on a computer somewhere under a user account. You build the website in that folder by logging in as that user.
DreamHost uses their own Panel accounts, but the Users and domains is pretty standard linux stuff. If you start using Linux as your OS, you will begin to understand why dreamhost has things set up the way they do.