How does transistor act as an amplifier....???

plzzz explain the diagram also....plzz

See the circuit diagram below.

The input circuit is forward biased. As a result, the resistance of input circuit is small. The output circuit is reverse biased and the resistance of output circuit is large.

The low input AC voltage signal is applied across emitter base circuit and amplified  AC voltage is obtained as change in collector voltage.

When no AC signal is applied

I_e=I_b+I_c

Where I_e =emitter current.

  I_b=base current.

   I_c=collector current.

If V_c is the collector voltage, then ,

V_CB=V_c+I_cR_c

Here R_c=load resistance.

Now let us explain the working. When positive half cycle of input AC voltage comes, it opposes the forward biasing of emitter base circuit. Due to this the emitter current decreases and consequently the collector current decreases. As a result, V_c increases or we can say that the output signal is amplified for the positive half cycle. This indicates during positive half cycle of input AC signal voltage , the output signal voltage at collector also varies through positive half cycle.

We can explain in a similar way for the negative half cycle of input AC signal voltage. Here the output signal voltage at collector also varies through the negative half cycle.