What PF is

Default packet filter (aka firewall with NAT) from OpenBSD 3.0 (December 2001)

Replaced IPFilter, which had to be removed due to licensing issues (which in turn lead to a license audit of the entire OpenBSD source tree)

Based on new code by Daniel Hartmeier (June 2001 ->), since hacked on by several others

High performance (see http://www.benzedrine.cx/pf-paper.html), low maintenance