Point-to-Point Protocol over L2TP
A Point-to-Point Protocol over L2TP connection uses PPP and L2TP server to establish the network.
PPP
Point-to-Point Protocol (PPP) is a protocol for providing a direct data link connection with authentication, encryption and compression.
L2TP
Layer 2 Tunneling Protocol (L2TP) is a protocol used to support VPNs, where security is provided in the transmitted packages rather than in the tunneling.