Transistor configurations refer to the different ways a transistor can be connected in a circuit to amplify or switch electronic signals. The three primary configurations are common emitter, common base, and common collector. Each configuration has unique characteristics that affect gain, input, and output impedance, making them suitable for various applications. In the common emitter configuration, the input signal is applied between the base and emitter, while the output is taken from the collector. The common collector configuration, also known as an emitter follower, provides high input impedance and low output impedance. Lastly, the common base configuration is less common but offers high-frequency response and low input impedance.