Every domain name that's registered and hosted has a great deal of DNS records - some of them are set up automatically when you host a domain, others can be set up by hand if needed. As an example, the A record of a domain is its IP address i.e. this record reveals where the web site is located; the MX records reveal where the emails are stored, while with an SRV record you can use a domain for unconventional purposes, including a Voice-over-IP service. Being able to set up and modify certain records allows you to have different services with different providers if, for instance, your present host provides good web hosting, but has problems with the email service, or you simply prefer to get a certain service from a particular company. Although you can create and modify these records freely and with no limits as you own a given domain address, a lot of companies charge extra to give you this option.