Each domain name that's registered and hosted has quite a lot of DNS records - some of them are created automatically when you host a domain, others can be created by hand if needed. For example, the A record of a domain address is its IP address i.e. this record reveals where the web site is located; the MX records show where the emails are stored, while through an SRV record you can use a domain name for unconventional purposes, including a Voice-over-IP service. To be able to set up and modify specific records allows you to have different services with different companies if, for instance, your current host provides stable web hosting, but has issues with the e-mail service, or you simply prefer to get a specific service from a specific company. While you can set up and modify these records freely and without any restrictions as you own a given domain name, a lot of companies charge extra to give you this option.