The word “hosting” doesn't describe a particular service, but a set of services which offer different functions to a domain. Having a website and e-mails, for instance, are two separate services though in the general case they come together, so a lot of people see them as one single service. In fact, every domain name has a couple of 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 defines where the website 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. For instance, an A record would be 220.127.116.11 and an MX record can be mx1.domain.com. Whenever you open a site 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 be forwarded to the correct server. The idea behind employing separate records is that the two services work with different web protocols and you could have your website hosted by one service provider and the e-mails by another.