The word “hosting” doesn't describe just one service, but a number of services which offer numerous functions to a domain name. Having a website and e-mails, for instance, are two separate services though in the general case they come together, so most of the people think of them as one single service. The truth is, every domain has a number of DNS records called A and MX, which show the server that deals with each particular service - the first one is a numeric IP address, that specifies where the website for the domain address is loaded from, while the latter is an alphanumeric string, which shows the server that deals with the e-mails for the domain name. For example, an A record can be 22.214.171.124 and an MX record is mx1.domain.com. Each time you open a site 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 directed to that company. If you have custom records on their end, the Internet browser request or the email will then be sent to the correct server. The concept behind employing separate records is that the two services employ different web protocols and you may have your website hosted by one service provider and the emails by another.