From DD-WRT Wiki
The Point-to-Point Tunneling Protocol (PPTP) is a method for implementing virtual private networks. PPTP has been made obsolete by Layer 2 Tunneling Protocol (L2TP) and IPSec, but its still commonly used and natively supported by a lage scale of routers and clients. 
For an alternative, have a look at the OpenVPN article.
A PPTP implementation may not fulfill enterprise class security requirements  but for home-use or for implementations that do not rely on high encryption grades should be sufficient. Just be sure to use strong passwords of >12 charaters. One of the big advantages of using PPTP over OpenVPN with DD-WRT is that PPTP is supported out-of-the-box for 4Mb firmware images and up.
 DD-WRT Resources
A good article explaining the implementation of a PPTP server for roadwarrior (aka remote) client access.
A article explaining the implementation of a PPTP bridge over the WAN for two or more DD-WRT routers.