Each domain name that is registered and hosted has lots of DNS records - some of them are created automatically when you host a domain, others can be created by hand if required. For instance, the A record of a domain is its IP address i.e. this record shows where the website is situated; the MX records show where the e-mails are managed, while using an SRV record you can use a domain address for unconventional purposes, including a Voice-over-IP service. The ability to create and modify particular records enables you to have different services with different providers if, for illustration, your present host provides good web hosting, but has problems with the e-mail service, or you simply prefer to get a certain service from a certain company. While you can set up and modify these records freely and without any restrictions as you own a given domain, a lot of companies charge extra to give you this option.