To put it in simple terms:
You buy a domain name. You can't do much with it UNLESS you have also purchased some website hosting space from a website hosting provider.
Once you have purchased a domain name and have some hosting space you can then upload your website files to the hosting space. The hosting provider will usually provide you with FTP/Control Panel usernames/passwords needed for uploading files and managing your website. Usually if you puchase the domain name and hosting space from the same company then they will point your domain name at their named servers.
If you purchase the domain name and the hosting space from different companies you might have the option to point your domain name at the hosting companies servers via the control panel yourself or your hosting provider may be able to do that for you, - most reasonable hosting companies will do that if you ask.