An SSL certificate is a file installed on the web server that a web site is running on. This file is composed of two segments. The first is the public key, which encrypts data. The second is the private key, which decrypts data. When a client, such as a web browser, connects to an SSL secured server a unique session is created using these two keys. All data transmitted during this session is protected, and cannot be decrypted by any party other than that specific client and the server.