The definition of “hosting” doesn't describe only one service, but a variety of services that provide various functions to a domain address. Having a site and emails, as an example, are two independent services although in the general case they come together, so a lot of people see them as one single service. Actually, each and every domain has a several 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 name is loaded from, while the latter is an alphanumeric string, which shows the server that manages the emails for the domain. As an illustration, an A record can be 123.123.123.123 and an MX record can be mx1.domain.com. Every time you open a website or send an e-mail, the global DNS servers are contacted to check the name servers that a domain has and the traffic/message is first directed to that company. In case you have custom records on their end, the web browser request or the e-mail will be forwarded to the correct server. The concept behind working with separate records is that the two services use different web protocols and you can have your website hosted by one company and the emails by another.