Every domain that's registered and hosted has quite a lot of DNS records - some of them are set up automatically when you host a domain, others can be set up manually if necessary. As an illustration, the A record of a domain address is its IP address i.e. this record shows where the site is located; the MX records show where the emails are stored, while through an SRV record you can use a domain address for unconventional purposes, such as a Voice-over-IP service. To be able to set up and modify specific records permits you to have different services with different companies if, for example, your present host provides stable web hosting, but has troubles with the e-mail service, or you simply prefer to get a certain service from a certain company. Though you can create and modify these records freely and without any limitations as you own a given domain address, many companies charge extra to give you this option.