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 created by hand if needed. For instance, the A record of a domain address is its IP address i.e. this record reveals where the web site is located; the MX records show where the e-mails are handled, while through an SRV record you can use a domain address for unconventional purposes, like a Voice-over-IP service. To be able to set up and modify certain records allows you to have different services with different companies if, for example, your present host provides stable web hosting, but has problems with the e-mail service, or you simply prefer to get a specific service from a certain company. While you will be able to set up and modify these records freely and without any limits as you own a given domain, some companies charge extra to give you this option.