Buying a hosting service is pretty much like buying a house. It is where you live, place all your valuables, your family and where your visitors would go.
So, your hosting service will house all the valuables and other collateral of your website such as photos, videos, sounds, write-ups and all other media items in place.
In cohorts to that, your domain will point all your visitors to all those materials. Your domain name is the address of the house.