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 needed. As an example, the A record of a domain name is its IP address i.e. this record shows where the web site is located; the MX records show where the emails are handled, while through an SRV record you can use a domain name for unconventional purposes, such as a Voice-over-IP service. The ability to create and modify certain records permits you to have different services with different companies if, for example, your current host provides good web hosting, but has troubles with the e-mail service, or you simply prefer to get a specific service from a certain company. Although you will be able to set up and edit these records freely and without any limitations as you own a given domain address, a lot of companies charge extra to give you this option.