The word “hosting” does not describe one service, but a number of services which offer a variety of functions to a domain. Having a website and emails, for example, are two separate services even though in the general case they come together, so a lot of people consider them as one single service. The truth is, each and every domain has a number of DNS records called A and MX, which show the server that manages each particular service - the former is a numeric IP address, which defines where the site for the domain name is loaded from, while the latter is an alphanumeric string, which shows the server that handles the e-mails for the domain. As an illustration, an A record would be 126.96.36.199 and an MX record would 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. When you have custom records on their end, the 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 employ different web protocols and you can have your website hosted by one service provider and the e-mail messages by another.