Secure Socket Layer (SSL) is a protocol developed by Netscape in 1996 which quickly became the method of choice for securing data transmissions across the Internet. SSL is an integral part of most Web browsers and Web servers and makes use of the public-and-private key encryption system developed by Rivest, Shamir, and Adleman.
SSL encrypts the segments of network connections at the Application Layer to ensure secure end-to-end transit at the Transport Layer. Several versions of the protocols are in widespread use in applications like web browsing, electronic mail, Internet faxing, instant messaging and voice-over-IP (VoIP).
In order to make a SSL connection, the SSL protocol requires that a server should have a digital SSL certificate installed. A digital SSL certificate is an electronic file that uniquely identifies individuals and servers. Digital SSL certificates serve as a kind of digital passport or credential which authenticate the server prior to the SSL session being established.
Typically, digital SSL certificates are signed by an independent and trusted third party to ensure their validity. The "signer" of a certificate is known as a Certification Authority (CA), such as VeriSign, thawte and GeoTrust.