Digital signatures and certificates...

a digital signature is basically a way to ensure that an electronic document is authentic it has not been tampered with. digital signatures rely on certain types of encryption to make sure that it is real.

a digital certificate is to do a public key encryption on a very large scale, it is basically a bit of information to say that the website is safe to use and trusted by an independant source called the Certificate Authority, it acts as a middle man that both parties can trust it shows that each computer knows who each other is then provides public keys to each other.

Public key
public key encryption is concidered very secure as it does not require a shared key between the sender and reciever. a "key" is a small piece of text code that triggers an algorithm to encode or decode text.