Each domain that's registered and hosted has lots of DNS records - some of them are created automatically when you host a domain, others can be created manually if necessary. For instance, the A record of a domain address is its IP address i.e. this record reveals where the web site is situated; the MX records show where the e-mails are stored, while through an SRV record you can use a domain name for unconventional purposes, like a Voice-over-IP service. Being able to set up and modify particular records enables you to have different services with different companies if, for instance, your present host provides good web hosting, but has troubles with the email service, or you simply prefer to get a particular service from a particular company. Although you will be able to create and modify these records freely and with no limits as you own a given domain name, many companies charge extra to give you this option.