A domain must be unique (for example there can only be one single Yahoo.com) and must be registered with a domain registrar (ie. NameCheap); while for sub domains, users can freely add it on top of the existing domain as long as their web host provide the service. Some would say sub-domains are the ‘third level’ domains in the sense that they are simply “sub folders” under the domain root directory, normally used to organize your website content in different languages or different categories.
Website hosting providers usually offer far more than just space on a server, many web hosts provide a full package to allow the website owner all the tools they require for their website. Most shared hosting plans come with a control panel which gives the user a simple to use interface to setup email address, add databases, FTP access to upload their website, backups, statistics, ecommerce shopping carts, and many scripts like WordPress blogging, or Joomla CMS. Many web hosting services also give you a free domain name, free advertising credits and a few other free bonuses so you choose their service.
Shared hosting is web hosting in which the provider houses multiple sites on a single server. For example, Site A shares the same server with Site B, Site C, Site D, and Site E. The upside is that the multiple sites share the server cost, so shared web hosting is generally very inexpensive. In fact, you can find an option for less than $10 per month.