A domain registrar is a company through which you purchase your domain name.
They charge an annual fee (sometimes prepaid for multiple years) for the leasing of your domain name.
If you fail to pay this annual fee, then your domain name will go into redemption for 60 days (during with time your website and email won’t work) before becoming available again for someone else to purchase.
Ubertor recommends using http://domains.ubertor.com as a domain registrar for cheap prices and full access to your domains.
Other examples of registrars are GoDaddy, Enom, VeriSign, and Tucows.