Alternatively referred to as an electron tube or valve and first developed by John Ambrose Fleming in 1904. The vacuum tube is a glass tube that has its gas removed which creates a vacuum. Vacuum tubes contain electrodes for controlling electron flow in early computers that used them as a switch or an amplifier. The picture shows a collection of different vacuum tubes to give you a better understanding.
Today, vacuum tubes are no longer used with computers and have been replaced by the transistor.