Each domain which is registered and hosted has a great deal of DNS records - some of them are created automatically when you host a domain, others can be created by hand if required. 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 e-mails are stored, while through an SRV record you can use a domain for unconventional purposes, like a Voice-over-IP service. Being able to create and modify specific records enables you to have different services with different companies if, for illustration, your present host provides stable web hosting, but has issues with the e-mail service, or you simply prefer to get a particular service from a specific company. Though you should be able to create and modify these records freely and with no limits as you own a given domain, a lot of companies charge extra to give you this option.