Just like you may go to many different services for your car, it takes many services to have a website. For your car, you go to one business to change the oil, another to work on your transmission and another to do body work. It is possible that one business could have people on staff to do all these tasks. It's the same for your website. The following is an explanation of the different technical areas of your website that may have a username and password.
Your domain name is a group of RedShelf characters that the internet, as a system, uses to locate your unique website on your web host. A person anywhere in the world can request your site, and find your site by following the directions associated with your domain name. You can move your website to a different web server, change the address associated with your domain name and know that people will still find your site.
Your domain name is not something that anyone but you should be able to access or change. So, you have an account with a domain registrar to keep other people from hijacking your domain name. If your domain name is registered through your web host, you may be able to make changes in your domain name account from the same place you make changes to your hosting account.
Your web hosting company will most likely give you a place where you can go online to pay your bill, change your billing address, add new domain names, add a database to your account, add or change email addresses, and many other functions. This is your control panel or your account. Some web hosts give you one place to do everything. Other web hosts split it up and give you two sets of usernames and passwords. The reason they do that is that you might have an office person take care of the accounting issues and a technical person take care of emails and databases, etc. If one person is doing both functions, that person may find it cumbersome to have to log into two different