Going back to the grandfather clock example, the pendulum will not swing infinitely as friction will cause the energy between swings to diminish. That is unless another force continues to add a little push to keep the oscillation going. Typically, this is done with a spring inside that coils and uncoils to push the pendulum a little bit on each swing, adding enough energy back in to keep it in motion.
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The essential components contained within the oscillator circuit act like the spring and pendulum to keep the frequency stable and continuous. These parts of an oscillator are a frequency determining circuit (such as a crystal in crystal oscillators), an amplifier, and a positive feedback circuit. The frequency circuit, as the name suggests, determines the frequency of the oscillator. Like the spring in the example, the amplifier and feedback circuit keep the current continuously flowing as it gets increased by passing through the amplifier and then gets fed back into the frequency circuit, allowing the oscillator to be self-sustaining.
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