A Domain Validated (DV) SSL certificate is the quickest and easiest way to secure a domain, as the Certificate Authority (CA) issuing the certificate only requires verification that the recipient actually owns the domain they wish to cover. This verification process can typically be completed in a matter of minutes. However, these certificates offer little in the way of SSL recognition, so they are recommended for websites where visitor trust is not of high importance and information like usernames, passwords, or credit card information is not required.
An Organization Validated (OV) SSL certificate requires a light vetting process by the Certificate Authority before being issued. These certificates are a nice middle-ground between DV and EV certificates, as they aren't as expensive as EV options but still offer more SSL and trust indications than basic between DV and EV certificates. These certificates typically take between 2-3 days to be issued.
EV stands for Extended Validation and is the most premium type of SSL certificate available. These certificates are identified on websites mainly by the green address bar, the most universally recognized symbol of trust on the web. EV certificates are becoming more and more commonplace in the industry, especially amongst ecommerce sites, as they are used by some of the most trusted sites in the world like Deutsche Bank, Twitter, Paypal, and more. These certificates require that a company complete a thorough vetting process before being issued.
Multi-domain or SAN (Secure Alternate Name) SSL certificates can cover multiple and different domain names on just one certificate. For example, Thawte SAN certificates can cover up to 25 different domains, whereas Comodo certificates can cover up to 100 domains with just a single SSL certificate. It's an easiest and cheaper solution than acquiring multiple single SSL certificates (one for each protected domain)