As explained earlier, a digital signature on a message consists of an encrypted hash of the message - first the message is hashed, and then the hash is encrypted using the private key of the signatory (usually a CA).
The recipient can check the signature by decrypting it using the signatory's public key, then hashing the message him/herself and comparing the two results.
Many methods of hashing are possible, combining different encryption algorithms with different hashing algorithms. One is particularly worth mentioning.