To simplify: a diode, aka rectifier in tube terms, has two pins - one lets in the positive and negative ac voltage and the other lets out only the positive (or negative). This sort of creates a DC current and is the first step in the power supply process of creating a DC voltage to be amplified.
The triode, or amplifer, has three pins. One lets in the audio ac signal, the second "takes in" in DC current (from the diode) and the third lets out the amplified signal. This works in the same fashion as a xerox machine copying and enlarging a document: the original document is the audio signal at the first pin, the blank copy paper is the second pin and the third pin is the copy paper in the output tray with the enlarged copy.
This is really overly simplistic as tubes have more pins than the prefix "di" "tri" etc imply. The other pins are used to let in current to heat the tubes in order for them to work, and as Marakanetz stated above, some tubes have more than one set of electrodes in the same single tube.
The triode, or amplifer, has three pins. One lets in the audio ac signal, the second "takes in" in DC current (from the diode) and the third lets out the amplified signal. This works in the same fashion as a xerox machine copying and enlarging a document: the original document is the audio signal at the first pin, the blank copy paper is the second pin and the third pin is the copy paper in the output tray with the enlarged copy.
This is really overly simplistic as tubes have more pins than the prefix "di" "tri" etc imply. The other pins are used to let in current to heat the tubes in order for them to work, and as Marakanetz stated above, some tubes have more than one set of electrodes in the same single tube.