Every single domain name that's registered and hosted has a great deal of DNS records - some of them are created automatically when you host a domain, others can be set up by hand if required. As an illustration, the A record of a domain address is its IP address i.e. this record shows where the site is located; the MX records show where the emails are managed, while with an SRV record you can use a domain for unconventional purposes, such as a Voice-over-IP service. Being able to set up and modify specific records permits you to have different services with different companies if, for instance, your present host provides stable web hosting, but has problems with the e-mail service, or you simply prefer to get a specific service from a specific company. While you will be able to create and edit these records freely and without any limitations as you own a given domain, some companies charge extra to give you this option.