VPN’s – SSL, PPTP & L2TP

I am trying to set up a VPN across a few different sites and remote access may be necessary.

Can anyone explain in detail what the difference is between SSL, PPTP and L2TP?

What is the difference between the layers of the OSI that they operate at (security-wise) and if they can be used together to create a more secure network, if not why not.

I am not quite understanding all of this as I initially thought that you would use SSL instead of say PPTP but I am unsure that this is the case and I can’t find any good resources to explain it on the internet.

Thank you in advance 🙂