PAP Password Authentication Protocol (PAP) is a password-based authentication protocol used by Point to Point Protocol (PPP) to validate users.
CHAP Challenge-Handshake Authentication Protocol (CHAP) authenticates a user or network host to an authenticating entity.
MS-CHAP MS-CHAP is the Microsoft version of the Challenge-Handshake Authentication Protocol, CHAP. The protocol exists in two versions, MS-CHAPv1 and MS-CHAPv2.
EAP - Extensible Authentication Protocol
EAP is an authentication framework, not a specific authentication mechanism. It is used in network and internet connections and defined in RFC 3748. It provides some common functions and negotiation of authentication methods called EAP methods. Each protocol that uses EAP defines a way to encapsulate by the user EAP messages within that protocol's messages.