Every domain name which is registered and hosted has a great deal of DNS records - some of them are created automatically when you host a domain, others can be set up by hand if necessary. For example, the A record of a domain address is its IP address i.e. this record reveals where the website is located; the MX records reveal where the e-mails are handled, while with an SRV record you can use a domain name for unconventional purposes, like a Voice-over-IP service. Being able to create and modify certain records allows you to have different services with different companies if, for example, your current host provides good web hosting, but has issues with the e-mail service, or you simply prefer to get a certain service from a certain company. Though you should be able to create and modify these records freely and without any limits as you own a given domain, many companies charge extra to give you this option.