PHP Captcha: What is a "private key" and "public key"?

What is the difference between priva开发者_运维知识库te key and public key and how is it used?

Public-key cryptography You create a key-pair (public and private) the receivers public key is used to encrypt messages. The private key of the receiver is used to decrypt the this message.

Normally private and public key are terms that belong to assymetric encryption algorithms. I don't see a connection to CAPTCHA or PHP5 in this special case.