Interesting app note from Silicon Labs about oscillators used on their wireless SoC series 2 devices, oscillator basics included. Link here (PDF)
An oscillator is an electronic circuit which generates a repetitive, or periodic, time-varying signal. In the context of Wireless SoC Series 2 devices, this oscillator signal is used to clock execution of instructions and peripherals in the device. For radio communication the oscillator also provides an accurate and low noise frequency reference to the transciever. There are multiple ways of generating such a signal, each with different properties that influence project cost, board size, and stability of the clock signal.