Each domain that's registered and hosted has a lot of DNS records - some of them are created automatically when you host a domain, others can be set up by hand if necessary. As an illustration, the A record of a domain address is its IP address i.e. this record shows where the site is located; the MX records reveal where the e-mails are stored, while with an SRV record you can use a domain for unconventional purposes, for instance a Voice-over-IP service. The ability to create and modify particular records allows you to have different services with different companies if, for illustration, your current host provides good web hosting, but has problems with the e-mail service, or you simply prefer to get a certain service from a certain company. Although you will be able to create and modify these records freely and without any limitations as you own a given domain address, lots of companies charge extra to give you this option.