Secure Sockets Layer
Secure Sockets Layer (SSL) is a security technology which utilizes an asymmetric encryption key pair to provide an encrypted link between a server and a remote device. Since a man-in-the-middle can still spoof certificates when a connection is established, a Certificate Authority (already known to the incoming client) typically verifies the public key of the server, to ensure a secure and direct connection.
While SSL was once considered a very secure protocol, it is no longer so. Although it is sometimes still used, it has largely been replaced with Transport Layer Security (TLS), which is stronger. Most servers still support SSL 3.0 (the latest although aged version), and sometimes even SSL 2.0 however, and will default to these older options if TLS connections are not supported or fail for other reasons.
Unfortunately, this has led to downgrade attacks, which trick the server into downgrading its security to SSL, which can be cracked. The attacker can then break the SSL encryption, and read communications which were intended to be secret.