A digital signature is a mechanism you can use to authenticate a message's sender or document's signer. Don't confuse signatures with certificates--they're significantly different. Digital signatures use public key technology to verify who signed electronic data and whether that data remains unchanged.
For email, digital signatures create a message digest by parsing the entire message through a hash algorithm. The message digest is typically a 128-bit to 256-bit number. The number is encrypted with the message sender's private key and added to the end of the message.
When the recipient receives and opens the message, the digital signature goes through the same hash algorithm to verify the sender and the message integrity using the sender's public encryption key.