**broken link removed** another good article -- crystal info starts on page 4.
An excellent listing of reference articles can be found **broken link removed**. Crystal oscillator article listings start about halfway down the page.
All oscillators work by positive feedback. All the crystal does is limit the frequency due to it's R-L C equivalent circuit. The crystal is an electro-mechanical system. When the crystal is deformed, it generates a voltage and when a voltage is applied, the crystal deforms. The crystal mechanically vibrates (oscillates) due to the positive feedback from the amplifier.