Each domain name that's registered and hosted has a great deal of DNS records - some of them are set up automatically when you host a domain, others can be set up manually if required. For instance, the A record of a domain address is its IP address i.e. this record shows where the web site is located; the MX records reveal where the e-mails are managed, while through an SRV record you can use a domain address for unconventional purposes, including a Voice-over-IP service. Being able to create and modify specific records enables you to have different services with different companies if, for example, your current host provides good web hosting, but has troubles with the e-mail service, or you simply prefer to get a specific service from a certain company. Although you should be able to create and modify these records freely and without any restrictions as you own a given domain, some companies charge extra to give you this option.