The definition of “hosting” doesn't describe a particular service, but several services that provide numerous functions to a domain address. Having a website and e-mails, as an illustration, are two individual services despite the fact that in the general case they come together, so many people consider them as one single service. In fact, each domain has a number of DNS records called A and MX, which show the server that handles each particular service - the former is a numeric IP address, which defines where the site for the domain address is loaded from, while the second one is an alphanumeric string, which shows the server that manages the e-mails for the domain name. As an illustration, an A record is 126.96.36.199 and an MX record would be mx1.domain.com. Each time you open a website or send an email, the global DNS servers are contacted to check the name servers that a Internet domain has and the traffic/message is first forwarded to that company. When you have custom records on their end, the browser request or the e-mail will then be sent to the correct server. The reasoning behind using separate records is that the two services work with different web protocols and you may have your website hosted by one service provider and the emails by another.