difference between crystal & oscillator
As others above have explained, an oscillator is a self-contained module that outputs a square wave at the desired frequency. A crystal needs a surrounding circuit to be useful, typically for PICs, a Pierce circuit with neighboring capacitors going to ground.
An oscillator is more expensive and draws more power, but doesn't depend on the accuracy of other components in the circuit.
**broken link removed** explains the trade-offs between crystals, oscillators, resonators, RC circuits, and others.
This explains the difference between all the crystal parameters you might see, like "Series vs Parallel", "Load capacitance," "Series Resistance," and more.
This
Microchip application note explains important design trade-offs in selecting crystals for PIC applications, including how the LP, XT, and HS PIC settings affect power use and startup ability.
**broken link removed** gives some abbreviated guidance for how the neighboring capacitors affect things and also explains PPMs.