Each domain 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 necessary. As an illustration, the A record of a domain is its IP address i.e. this record shows where the web site is located; the MX records reveal where the e-mails are handled, while using an SRV record you can use a domain address for unconventional purposes, for instance a Voice-over-IP service. Being able to create and modify specific records enables you to have different services with different providers if, for illustration, 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. Even though you will be able to set up and edit these records freely and with no limits as you own a given domain address, some companies charge extra to give you this option.