P-Brick is an acronym for a programmable brick. Lego do a number of these:

These are part of Lego’s Mindstorms range. They allow a user to do robotics experiments through being able to program and interface these bricks. Most listed will work with NQC.

It is worth noting that many of the construction toys are now producing these as well as Lego. FischerTechnik have a very interesting system.

The important aspects of a P-Brick are reusability, easy structural and electronic interfacing, a decent array of sensors and actuators in the system, and a progressive programming system - that is - a simple one to get kids started, and an advanced one like NQC for more advanced tasks.