An SSL certificate protects sensitive data by encrypting the data when sent across the network.
This Secure Socket Layer protocol allows a secure connection between the sending and receiving party and prevents unauthorized access by those who have malicious intent.
There are various types of SSL certificates available:
1. Extended validation
2. Organization validation
3. Domain Validation
For further details about the above, kindly navigate to http://www.hostingadvice.com/blog/ssl-certificates-explained-made-easy/