Domain Registrars are what you use to to register a domain name. You need a domain name in order to create a website. It serves as the virtual address of your site. Among the things to consider when choosing which domain registrar to use are:
1) Price (factoring any freebies that you plan on using that are included)
2) Other services offered (can you use them for your other website needs)
3) Customer support
4) Special perks (auto renewal, email updates, domain protections...)
Top Picks (My current registrar is Go Daddy)
For the complete list of ICANN (Internet Corporation for Assigned Names and Numbers) accredited registrars go to http://www.icann.org/en/registrars/accredited-list.html.