A tank circuit is typically a combination of 'capacitive' and 'inductive' circuits that exchange 'energy' back and forth. An inductor stores energy as a magnetic field, wheras a capacitor stores energy as a charge across plates. Ideally, the exchange between them would go on forever (back and forth like a game of hot potato) however, a slight replenishing signal is needed practically. By selecting the proper values of components, the rate, or frequency, of exchange can be determined.