From the Linksys WRT54G

Virtual Private Networking (VPN) is typically used for work-related networking. For VPN tunnels, the Router supports IPSec Pass-Through and PPTP Pass-Through.

* IPSec - Internet Protocol Security (IPSec) is a suite of protocols used to implement secure exchange of packets at the IP layer. To allow IPSec tunnels to pass through the Router, IPSec Pass-Through is enabled by default. To disable IPSec Pass-Through, uncheck the box next to IPSec.
* PPTP - Point-to-Point Tunneling Protocol is the method used to enable VPN sessions to a Windows NT 4.0 or 2000 server. To allow PPTP tunnels to pass through the Router, PPTP Pass-Through is enabled by default. To disable PPTP Pass-Through, uncheck the box next to PPTP.