What is the difference between serial and parallel communication?

Serial communication transmits data one bit at a time over a single wire (plus ground/clock), requiring fewer connections but lower speed. Parallel communication transmits multiple bits simultaneously over multiple wires, offering higher speed but more connections and synchronization challenges. Serial examples: UART, SPI, I2C, USB. Parallel examples: Parallel port, memory buses. Modern systems prefer serial for external interfaces (fewer pins, longer distances, lower EMI) while using parallel internally for speed.