Because SSL is built into all major browsers and web servers, simply installing a digital certificate turns on their SSL capabilities.

There are two certifications :

Server certification
  Authenticate your site. A digital certificate on your server automatically     communicates your site's authenticity to visitors' web browsers,     confirming that the visitor is actually communicating with you, and not     with a fraudulent site.

 Keep private communications private.
  Digital certificates encrypt the data visitors that exchange with your site   to keep it safe from interception or tampering using SSL (Secure   Sockets Layer) technology, the industry-standard method for   protecting web communications

Personal certifications:
 Requiring registration and passwords lets you keep track of visitors, but  if exchanging this information isn't secured, data can be intercepted  online. Fortunately, personal certificates can help. Not only do digital  certificates help you secure e-commerce transactions with visitors, they  can also help you learn more about your customers and enhance their  online experience.

 Instantly authenticate a visitor's identity, instead of requiring individual  usernames and passwords.

 Different companies like VeriSign etc assign certification.