Every single domain that 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 created manually if needed. As an illustration, the A record of a domain address is its IP address i.e. this record reveals where the site is located; the MX records show where the e-mails are stored, while with an SRV record you can use a domain for unconventional purposes, including a Voice-over-IP service. Being able to set up and modify particular records allows you to have different services with different providers if, for instance, your current host provides dependable web hosting, but has troubles with the e-mail service, or you simply prefer to get a particular service from a particular company. Though you should be able to create and modify these records freely and with no restrictions as you own a given domain name, lots of companies charge extra to give you this option.