Every domain which is registered and hosted has quite a lot of DNS records - some of them are created automatically when you host a domain, others can be set up manually if necessary. 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 reveal where the emails are stored, while through an SRV record you can use a domain address for unconventional purposes, for instance a Voice-over-IP service. Being able to set up and modify specific records allows you to have different services with different providers if, for example, your present host provides stable web hosting, but has troubles with the email service, or you simply prefer to get a certain service from a particular company. Though you should be able to create and modify these records freely and with no limits as you own a given domain, lots of companies charge extra to give you this option.