Each and every domain that is registered and hosted has a lot of DNS records - some of them are created automatically when you host a domain, others can be set up by hand if necessary. As an example, the A record of a domain address is its IP address i.e. this record reveals where the site is located; the MX records show where the e-mails are stored, while using an SRV record you can use a domain address for unconventional purposes, including a Voice-over-IP service. Being able to create and modify certain records allows you to have different services with different companies if, for illustration, your present host provides stable web hosting, but has problems with the email service, or you simply prefer to get a specific service from a certain company. While you can create and edit these records freely and with no limits as you own a given domain, lots of companies charge extra to give you this option.