What is does a transistor do?

It is basically an electronic "thing" that has three terminals. Lets say E (emitter), B (base) and C (collector). Lets say we connect C to positive of a battery and E to negative (or ground). As long as we keep some current at B , we'll have current flowing from C to E . Basically its some device that controls current on one side from current on another side. the former being larger in magnitude than the later.
ANSWER: Is a semiconductor voltage amplifier