The term “hosting” does not describe a particular service, but a set of services that provide numerous functions to a domain. Having a site and emails, for example, are two independent services although in the general case they come together, so most of the people see them as one single service. Actually, every single domain has a couple of DNS records called A and MX, which show the server that handles each specific service - the first one is a numeric IP address, which identifies where the website for the domain is loaded from, while the second one is an alphanumeric string, which shows the server that manages the e-mails for the domain address. For example, an A record is 22.214.171.124 and an MX record can be mx1.domain.com. Each time you open a website or send an email, the global DNS servers are contacted to check the name servers that a domain has and the traffic/message is first forwarded to that company. When you have custom records on their end, the web browser request or the e-mail will then be directed to the correct server. The concept behind working with separate records is that the two services work with different web protocols and you can have your site hosted by one provider and the emails by another.