What is an SSL certificate?
SSL is a cryptographic protocol used for a more secure communication between two nodes (computers, servers) on the internet.
SSL uses asymmetric cryptography to validate and authenticate exchange keys, and symmetric encryption to maintain confidentiality.
To verify authentication, and ownership of the public key part to its real owner, SSL uses certificates. It is best to use certificates issued by a certification authority, but it is possible to use self-signed certificates.
This protocol is widely used for instant messaging, VoIP calls, as well as receiving and sending mail.
Even for transferring data from some sites to your computer, HTTPS (Hypertext Transfer Protocol Secure) is used, which in turn uses SSL, thereby protecting data.
If you see a green icon in your browser's bar that notifies you that the connection is encrypted - be aware that your data is safe with a very high probability.