This applet models the interaction between electrons (shown as blue dots) and photons (red squiggles).
This applet is based on the Feedback applet; the text accompanying that applet will explain the purpose of all the fields, save for the decay rates. The biggest difference is simply the addition of an extra two energy levels.
Absorption and emission take place on the centre two energy levels. There is also a probability (the "lower decay rate") that an electron will decay from the lower of these two levels to the lowest level. No interaction is possible at this state, and the electron will remain here until pumping (set by the "pump rate") moves it to the topmost energy level. Once on the highest energy level, it will remain here until it decays to the level immediately below (probability given by the "upper decay rate").
Interaction between electrons and photons is not possible on the highest or lowest energy level; pumping and decay are the only mechanisms by which an electron may move away from these levels.