An asymmetric system has one major disadvantage - the algorithms are very complex and therefore very time-consuming. No matter how fast your computer is, you wouldn't want to use them to encrypt/decrypt large amounts of data.
So encryption software normally uses a symmetric algorithm on the data itself, meaning the secret key has to be sent to the recipient - but in case an eavesdropper intercepts the secret key, the software sends the secret key encrypted, using an asymmetric algorithm.
In this kind of system, the secret key is called a session key, since it is generated when the would-be sender and recipient first connect, sent using the asymmetric algorithm, and then used for all the data transmissions until the session is complete and the connection broken. A new secret key is generated for every new session.