A complex airplane has "an airplane that has a retractable landing gear, flaps, and a controllable pitch propeller". This page collects together som articles on the systems in complex aircraft.
From McCauley - a series of pages: http://www.mccauley.textron.com/prop/prop-tech/pg00intro.html
From AvWeb's John Deakin: http://www.avweb.com/news/columns/182082-1.html
From AvWeb's John Deakin: http://www.avweb.com/news/columns/182081-1.html