Domain Validated (DV)
This is the most common type of SSL certificate. You will be required to upload a file to your site to prove you own it, or in some cases this can be done by matching your email address against the publicly held whois information.
Organisation Validated (OV)
This type of certificate provides further trust to website visitors by verifying the organisation itself. For companies, this may mean cross referencing with Companies House. For individuals, you may be required to supply a copy of your driving license or other documents. The organisations name is then listed in the certificate itself.
Extended Validation (EV)
On top of the steps taken for an OV certificate, further checks are made to verify existence of the organisation. This is the gold standard in SSL certificates and, once completed, enables additional trust indicators like the fully green browser address bar.