Every domain name 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 created by hand 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 situated; the MX records show where the e-mails are handled, while through an SRV record you can use a domain name for unconventional purposes, including a Voice-over-IP service. Being able to create and modify particular records allows you to have different services with different providers if, for illustration, your current host provides stable web hosting, but has troubles with the email service, or you simply prefer to get a particular service from a particular company. Although you should be able to set up and edit these records freely and without any limitations as you own a given domain, a lot of companies charge extra to give you this option.