Vacuum tubes are electronic devices that control the flow of electric current in a vacuum. They consist of a sealed glass or metal envelope containing electrodes, including a cathode and an anode. When heated, the cathode emits electrons, which travel through the vacuum to the anode, allowing the tube to amplify signals or switch currents. Originally developed in the early 20th century, vacuum tubes were essential in early radios, televisions, and computers. They have largely been replaced by transistors in modern electronics, but they are still used in some high-fidelity audio equipment and radio frequency applications due to their unique sound characteristics.