What is an oscillator and what conditions are required for oscillation?

An oscillator is a circuit that generates a periodic waveform without an external input signal, converting DC power to AC. For sustained oscillation, the Barkhausen criteria must be met: the loop gain must be exactly unity (|A*beta| = 1), and the total phase shift around the loop must be 0 degrees or a multiple of 360 degrees. Common types include RC, LC, and crystal oscillators.