What kind of relay changes electrical current and how?

as an example, If I were to make a small electric motor turn right or left what kind of relay device would switch the current from + - to - + and make the motor rotate from clockwise to counterclockwise. Post a link on how this works if necessary. In other words, how does an RC car go backwards by switching the... show more as an example, If I were to make a small electric motor turn right or left what kind of relay device would switch the current from + - to - + and make the motor rotate from clockwise to counterclockwise. Post a link on how this works if necessary. In other words, how does an RC car go backwards by switching the current on the motor? Does the relay even do this?
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