What are the two main uses of a transistor?

a) to act as a switching device; a change in the bias voltage at the base- emitter junction can cause an increase in signal flowing through the transistor through the collector terminal

and this cause the output voltage at the collector terminal to change; eg to drop to a low voltage level, this is seen as the transistor device switching on to maximum conduction rate and a low level output at the collector.

b) to act an amplifer . whether the input signal is an ac type signal in which case its a signal power amplifer function; eg sound amplifer control circuits , or whether a dc input signal, in which case the input dc signal is amplfied to be reproduced as a bigger signal at the output collector terminal