Each and every domain which is registered and hosted has a lot of DNS records - some of them are set up automatically when you host a domain, others can be set up manually if required. For instance, 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 e-mails 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 specific records enables you to have different services with different companies if, for example, your present host provides good web hosting, but has problems with the e-mail service, or you simply prefer to get a certain service from a specific company. Even though you should be able to create and modify these records freely and with no limitations as you own a given domain address, lots of companies charge extra to give you this option.
