Difference between a diode and a transistor?

Answer 1

A diode in simple form is a single PN junction (negatively doped material comprises the cathode and postively doped material the anode),  electrical current flows N to P when the cathode becomes more negative or the anode becomes more positive. A transistor in simple form is made of two junctions and can be arranged NPN or PNP (emitter-base-collector). Current flows emmiter to collector as determined by the polarity of the voltage felt on the base. There is some emitter to base current flow depending on biasing, but that's a question to itself.