The definition of “hosting” doesn't describe only one service, but a variety of services that offer numerous functions to a domain. Having a site and emails, as an illustration, are two individual services though in the general case they come together, so most people think of them as one single service. The truth is, every domain has a several DNS records called A and MX, which show the server that deals with each particular service - the former is a numeric IP address, that defines where the website for the domain is loaded from, while the latter is an alphanumeric string, which shows the server that manages the e-mails for the domain name. As an example, an A record can be 126.96.36.199 and an MX record is mx1.domain.com. Each time you open a website or send an e-mail, the global DNS servers are contacted to check the name servers that a Internet domain has and the traffic/message is first directed to that company. If you have custom records on their end, the web browser request or the email will be directed to the correct server. The reasoning behind working with separate records is that the two services use different web protocols and you may have your site hosted by one service provider and the e-mail messages by another.