Multiple domain hosting generally refers to hosting several domain names in one single hosting account, and with options to managed all the domain names through a single hosting account control panel.
How does these multiple domain setup back end ?
There are different approach to setup multiple domain in various systems, in a typical cpanel hosting environment, cpanel is one of the most popular control panel in web hosting offerings, the approach is to assign a master domain to a hosting account, and then setup additional domains on to the master domain, where the addon domain is basically parking to a subdomain of the master domain, and that the root path for the addon domain is pointing to a subfolder within the public_html folder of the master domain hosting account.
This setup approach allows the hosting account user to serve unique set of content for each of the domain in the hosting account, as well as able to manage all domain related task such as setting up email accounts for the addon domain through the cpanel hosting account control panel.
Do my website visitors notice the difference ?
Website visitors to each of the domain hosting in the same hosting account will not notice the difference, surfing experience will just be like accessing any website on the internet, only the webmaster who owns and setup the domains will know these domains are actually setup all in one single hosting account. This type of multi domain account is common and popular among webmasters that manages several domains of their own, a typical way of budget hosting account offer.