Wow, two clicks through Wikipedia and i get this:
and a whole lot more.
That’s a pretty interesting photo there, staplermofo. From what I remember from uni, stupido (betraying his namesake) has it right. The CPL in your hand converts the linearly polarized light from the LCD into circularly polarized light. The linear polarizer on your lens (a) filters all linear light from the LCD and (b) takes out half the circularly polarized light coming through the CPL, but half still comes through.
A decent picture of a linear polarizer can be found on Wikipedia’s “polarizer” entry. The wire grid may seem oriented wrong, but it is actually correct.
Be careful mounting your linear polarizer on your (D)SLR – the internal focusing and metering optics of the SLR don’t work well with linears (but do work just fine with CPLs).