The word “hosting” doesn't describe just one service, but several services which offer various functions to a domain address. Having a website and emails, as an illustration, are two individual services though in the general case they come together, so most people consider them as one single service. In reality, every single domain has a several DNS records called A and MX, which show the server that manages each particular service - the first one is a numeric IP address, which specifies where the website for the domain name is loaded from, while the latter is an alphanumeric string, which shows the server that handles the emails for the domain name. As an illustration, an A record can be 126.96.36.199 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 domain address has and the traffic/message is first directed to that company. If you have custom records on their end, the browser request or the email will then be directed to the correct server. The idea behind using separate records is that the two services work with different web protocols and you may have your site hosted by one company and the emails by another.