What is an SSL security certificate? What does it do?

SSL (Secure Socket Layer) protocol is used to ensure secure transactions between web servers and browsers. This protocol uses a third party, a Certificate Authority (CA), to identify one end or both end of a transaction. An SSL certificate binds together a domain name with an organizational identity and location. When installed on a web server, SSL certificates activate certain protocols allowing secure connections from a webserver to a browser. They are typically used to secure credit card transactions, data transfer and logins, and more recently securing browsing of social media sites.

SSL (Secure Sockets Layer) is a standard security technology for establishing an encrypted link between a server and a client—typically a web server (website) and a browser; or a mail server and a mail client (e.g., Outlook).

SSL allows sensitive information such as credit card numbers, social security numbers, and login credentials to be transmitted securely. Normally, data sent between browsers and web servers is sent in plain text—leaving you vulnerable to eavesdropping. If an attacker is able to intercept all data being sent between a browser and a web server they can see and use that information.

I guess everybody answers it already.

But in layman's term... SSL Certificate identify and make sure that the site you're visiting is authenticated and that the connection is secured at least in the eyes of common people. But for security experts or hacker, it's a different story in some cases.

In a common language SSL (Secure Socket Layer) is like a tunnel which transmit your data securly. It ensure uninterruptable reaching of data to the destination. The reason of this confidence is the cryptographic mechanism of data encryption and multiple layers of verifying the integrity of data. 

A very small but quite reasonable definition of the SSL is given on link below, where you will also have a basic concept of SSL certificate and how can anyone acquire this certificate.

http://www.digicert.com/ssl.htm

SSL stands for Secure Socket Layer. 

To secure the information is transmited, SSL encryptions works by mixing the real info with fake data. It adds layers of random data to the unencrypted data. In order to be able to read the real data you need to know which is fake. Intercepting a transmision between two parties would get you a lots of scrambled data. Making it impossible to distinguish between the real and fake one. 

SSL is a protocol that operated directly on the top of TCP.This way protocols on higher layers(such a HTTP ) can be left unchangedwhile still providing a secure connection underneath the SSL layer HTTP is identical to HTTPS. So when SSL/TLS correctly all an attacker can see on the cable is which IP and domain you are connected to roughly how much data you are sending and what encryption and compression is used. He can also terminate the connection ,but both sides will know the connection is interuppted by third party

What is a SSL Certificate? 

A SSL (Secure Socket Layer) Certificate is a file containing the "fingerprint" of your server and your domain name.

What does the SSL Certificate do? 

The SSL certificate allows webpages to be loaded in a secure, encrypted environment. You can tell if a webpage is being displayed in a secure environment because the "http://" in the web address will be replaced with "https://". When a webpage is loaded in a secure "https://" environment, the page content is encrypted by the server before it is displayed on-screen, so that it cannot be read by third parties.