A domain name registrar is the company from which you will purchase your domain name. Your domain is simply the "sign on the door" of your website. It is unrelated to the actual content of your site until we link your domain to your web host's name servers.
Popular Domain Name Registrars include GoDaddy, NameCheap, and 1and1. Usually, you'll be able to find a domain name for around $10-$12 per year.