A web hosting service is a type of Internet hosting service that allows individuals and organizations to make their website accessible via the World Wide Web. Web hosts are companies that provide space on a server owned or leased for use by clients, as well as providing Internet connectivity, typically in a data center. Web hosts can also provide data center space and connectivity to the Internet for other servers located in their data center, called colocation, also known as Housing in Latin America or France. (Source: wikipedia)
Domain registration is the process of registering a domain name, which identifies one or more IP addresses with a name that is easier to remember and use in URLs to identify particular Web pages. The person or business that registers domain name is called the domain name registrant.
Domain registration requires utilizing the services of a domain name registrar, an ICANN or national ccTLD accredited company that has the authority to register domain names. Registrars help individuals and organizations register a domain name that has an extension like .com, .org, .net, .info, .biz, .us, .mobi, .name, .pro, .tv, etc.
(Source: webopedia definition)