The term “hosting” does not describe only one service, but a set of services which provide a variety of functions to a domain. Having a website and e-mails, for instance, are two independent services though in the general case they come together, so a lot of people consider them as one single service. In fact, each and every domain has a couple of DNS records called A and MX, which show the server that handles each particular service - the former is a numeric IP address, that defines where the site for the domain name is loaded from, while the latter is an alphanumeric string, which shows the server that deals with the emails for the domain address. As an illustration, an A record can be 126.96.36.199 and an MX record can be mx1.domain.com. Every time you open a website or send an email, the global DNS servers are contacted to check the name servers that a domain name has and the traffic/message is first directed to that company. When you have custom records on their end, the browser request or the e-mail will be forwarded to the correct server. The idea behind using separate records is that the two services employ different web protocols and you can have your website hosted by one company and the emails by another.