Domain Validation certificates authenticate that the purchaser of the cert also owns (or administers) the domain name to which the cert will be applied. This lowest level of authentication also offers the lowest prices and allows you to secure your web site the most rapidly.
Organization Validation certificates receive some scrutiny to confirm the relationship between the domain name, domain owner, and the business for which the certificate will be used.
Extended Validation certificates require the certificate authority (the company issuing the certificate, such as GeoTrust or Symantec) to perform a more extensive verification of the purchaser's business and their authority to purchase a certificate on behalf of that company. Once installed, EV certificates turn the browser address bar green to give visitors a visual cue that this is a trustworthy site.