The word “hosting” doesn't describe just one service, but a number of services that offer a variety of functions to a domain name. Having a website and emails, as an illustration, are two independent services although in the general case they come together, so most people consider them as one single service. Actually, 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, which defines 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 instance, an A record can be 18.104.22.168 and an MX record is 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 forwarded to that company. If you have custom records on their end, the web browser request or the email will then be forwarded to the correct server. The reasoning behind using separate records is that the two services work with different web protocols and you can have your site hosted by one provider and the e-mails by another.