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general purpose npn transistors can be used in wide variety of applications but generally in low power area in order to condition the control signal. power transistor is generally used to deal with high power applications like smps etc. capable of handling large current and voltages so high power.
A power transistor is capable of handling much more current than a general transistor. This is why power transistors have metallic holes on top. It is to attach a heat sink so that it can dissipate excess power, since power transistors are capable of handling so much power, it needs a way to dissipate the excess amount.