SSL defines a layer in the TCP / IP model. His job is to protect the data transmitted on the network. This protection is achieved by encrypting all communications between the connection endpoints. By embedding SSL in the layer model, it can be used for different overlying protocols. The best known of these protocols are HTTP, FTP and they are mostly indicated by appending an 'S' (HTTPS).
An important element in securing through the use of SSL are certificates. These enables participants to authenticate each other and thus prevent the eavesdropping by third parties (MITM - Man in the Middle).