That’s a bit of a tricky question, since VPN protocols can have different settings and can encapsulate each other. But let’s try to count them one by one.
1) PPTP most popular and widely supported protocol, has 128-bit encryption and quite fast comparing to other protocols. But not so well protected, since encryption key of 128-bits is too short for today.
2) L2TP is a protocol which does not imply encryption an usually used over IPSec protocol to secure data transfer. L2TP supports authentication in a very similar way as PPTP, but has no encryption. IPSec provides high industry standard encryption and even certificate authentication. L2TP rarely used without IPSec
3) SSTP is L2TP protocol over SSL connection.
3) OpenVPN – open standard and open source implementation. Supports different kinds of encryption. Advantages: can use TCP/IP to pretend SSL connection. Can bypass HTTP proxy. High level of protection, but VERY slow and requires a lot of processing power comparing to other protocols.
4) Private VPN implementation (for instance CISCO)