What is the difference between SSL VPN and IPSec VPN?

The IPSec is a set of protocols which operate on a network layer of the OSI Model - it protects the data sent between two endpoints by encrypting the IP traffic. Generally, the IPSec requires a dedicated hardware and/or software ("client" software) and specific knowledge to configure it properly and therefore is quite expensive to implement. SSL VPN is based on the SSL (secure socket layer) protocol - virtually every computer nowadays supports it. That means that your computer already has the "client" software to access the SSL VPN. Traditionally SSL VPN was associated with web-browsers (so you could use it only for a web-based traffic) - however with solutions like OpenVPN you can now create a VPN solution quite similar (and equally secure) to the one offered by IPSec.