A form of Encryption
When sending a message, the sender uses a message-digest algorithm to generate a shorter version of the message that can be encrypted, called a message digest. Next the sender uses their private key to encrypt the message digest. The sender transmits the message and the encrypted message digest to the recipient.
Upon receiving the message, the recipient decrypts the message digest. The recipient uses the same hash function on the message to generate another message digest, and compares the decrypted message digest against the newly generated one.
If the message digests are identical, the recipient knows that the message was indeed sent by the person claiming to be the sender and that the message was not modified during transmission.
If the message digests differ, the recipient knows that either the message was sent by someone else claiming to be the sender or that the message was modified or damaged during transmission.
The encrypted message digest serves as a digital signature for the message. The signature verifies the identity of the sender and the contents of the message.
Also, DIgest is a secure one way hash that can't be decrypted, for web sites and TCP/IP traffic. Both sides use the algorithm to encrypt the name and password and compare the result. Banks and Online Brokerage sites use this.