Every single domain which is registered and hosted has lots of DNS records - some of them are created automatically when you host a domain, others can be created by hand if required. As an illustration, the A record of a domain is its IP address i.e. this record reveals where the web site is situated; the MX records show where the e-mails are managed, while with an SRV record you can use a domain address for unconventional purposes, including a Voice-over-IP service. Being able to set up and modify specific records permits you to have different services with different providers if, for example, your current host provides stable web hosting, but has troubles with the e-mail service, or you simply prefer to get a particular service from a specific company. Although you should be able to set up and modify these records freely and with no limitations as you own a given domain name, many companies charge extra to give you this option.