By how it's biased. For AC amplification the transistor is typically biased with the collector voltage at 1/2 the supply voltage. The AC signal to the base then causes the transistor current to increase and decrease, which appears across the collector resistor as an amplified version of the input AC signal. The amplification results from the transistor gain between input voltage/current to output current.
For amplifying DC signals the transistor are usually configured in a feedback loop to stabilize the circuit, such as an op amp. If used as a DC switch then the transistor is simply turned completely on or completely off.
The reason for me asking is that I have this confusing mulitiple choice question it says- One MAIN function of a transistor is to, A. conduct in reverse direction at a certain voltage level. B. amplify a.c or d.c signals , C provide a variable amplitude a.c signal, D. vary the circuit resistance, the best choice for me was B. So I'm just making sure I'm correct..