(This is supplmental material for the article "Email Shenanigans: Who Really Requested That Clinical Trial Info?")

Identity Authentication — the act of confirming the true identity of an entity or person via a variety of means of verification.

Digital Identity — usually taken to mean the online equivalent of an individual human being, organization, company, or electronic device which participates in electronic transactions on behalf of the entity, i.e. person or company in question.

Digital Signature (not to be confused with a digital certificate) — an electronic signature that can be used to authenticate the identity of the sender of a message or the signer of a document, and possibly to ensure that the original content of the message or document that has been sent is unchanged. 1

Digital Certificate — an electronic document which uses a digital signature to bind a public-key with an identity.

Hashing — the transformation of a string of characters into a usually shorter fixed-length value or key that represents the original string and is used to index and retrieve items in a database.

Public Key Cryptography — a cryptographic system requiring two separate keys, one to lock (encrypt) plaintext, and to unlock (decrypt) cyphertext. 

A digital signature can be used with any kind of message, whether it is encrypted or not, simply so that the receiver can be sure of the sender’s identity and that the message arrived intact. A digital certificate contains the digital signature of the certificate-issuing authority so that anyone can verify that the certificate is real.

Source - www.searchsecurity.techtarget.com