Domain registrars are companies that lease domain names for certain periods of time. When a domain name is bought, it is actually registered with a domain registrar, like GoDaddy.com. The domain registrar has access to plenty of DNS servers that access the Domain Name System.
When you lease a domain name from a registrar, you have complete control over which server the domain name is hosted on, which means you can then associate that domain with your new web hosting account. Most major web hosting providers are also domain registrars as well.
Domain registrars often have the same or similar prices for all of their domains, with .com domains costing around $10 and .info domains costing about $1. Many domain registrars offer discounts when more than one domain is bought simultaneously.