Every domain which is 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 set up by hand if necessary. As an example, the A record of a domain name is its IP address i.e. this record reveals where the site is situated; the MX records show where the emails are managed, while using an SRV record you can use a domain for unconventional purposes, like a Voice-over-IP service. The ability to set up and modify certain records allows you to have different services with different providers if, for example, your current 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 should be able to set up and edit these records freely and without any limitations as you own a given domain name, lots of companies charge extra to give you this option.