L2TP or Layer 2 Tunneling Protocol is an extension to the PPP protocol that merges the best features of two other tunneling protocols PPTP and L2F, and is used to support virtual private networks (VPNs). As L2TP does not provide confidentiality or strong authentication by itself, it is often implemented along with the IPSec protocol to secure the connection. The combination of these two protocols is generally known as L2TP/IPsec.
L2TP/IPsec encapsulates data twice, which can slow down the connection. However, the protocol makes this up by providing the encryption/decryption process in the kernel, and allowing multi-threading that is not possible in the OpenVPN protocol. This fact, theoretically, makes L2TP together with IPsec faster than OpenVPN.
L2TP/IPsec is just as easy and quick to set up as PPTP, but is much more secure at the cost of slightly reduced speed. Yet, because the L2TP protocol uses UDP port 500, there are chances that the VPN connection will be detected and blocked by some firewalls.