Each domain name that is registered and hosted has a lot of DNS records - some of them are created automatically when you host a domain, others can be created manually if necessary. As an example, the A record of a domain name is its IP address i.e. this record shows where the site is situated; the MX records show where the e-mails are managed, while using an SRV record you can use a domain for unconventional purposes, like a Voice-over-IP service. To be able to create and modify particular records permits you to have different services with different companies 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 certain company. While you can create and modify these records freely and without any limitations as you own a given domain address, many companies charge extra to give you this option.