RSA is combined with a secret-key cryptosystem, such as DES (see Question 64), to encrypt a message by means of an RSA digital envelope.
Suppose Alice wishes to send an encrypted message to Bob. She
first encrypts the message with DES, using a randomly chosen DES
key. Then she looks up Bob's public key and uses it to encrypt
the DES key. The DES-encrypted message and the RSA-encrypted DES
key together form the RSA digital envelope and are sent to Bob.
Upon receiving the digital envelope, Bob decrypts the DES key
with his private key, then uses the DES key to decrypt to message
itself. This combines the high speed of DES with the key-management
convenience of RSA.