The word “hosting” doesn't describe just one service, but several services that offer different functions to a domain address. Having a website and emails, for instance, are two independent services although in the general case they come together, so a lot of people consider them as one single service. The truth is, every single domain name has a number of DNS records called A and MX, which show the server that deals with 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 address. As an illustration, an A record can be 220.127.116.11 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 Internet domain has and the traffic/message is first forwarded to that company. In case you have custom records on their end, the web browser request or the e-mail will then be forwarded to the correct server. The idea behind using separate records is that the two services employ different web protocols and you may have your site hosted by one company and the e-mails by another.