Hi friend......
Transistor is an electronic device, the name itself TRANSfer of resISTOR i.e., the transfer of current or voltage from high resistance to low resistane. (TRANSISTOR= TRANSfer+ resiSTOR).
Coming to its construction, transistor has three plates or terminals namely emitter, collector, base.
Emitter: Moderate size and heavily doped
Base: Small size and less doped
Collector: Big size and moderately doped
Transistor has two junctions namely Base-Emitter junction and Base-collector junction.
For NPN transistors, electrons are majority carriers and holes are minority carriers
For PNP transistors, holes are majority carriers and electrons are minority carriers
http://media-1.web.britannica.com/eb-media/28/76428-004-57D480F6.gif
http://www.radartutorial.eu/21.semiconductors/pic/transistor9.en.jpg
Coming to its operation, I consider NPN transistor shown in above figures. If I apply forward bias to Base-emitter junction and reverse operation to Base-collector junction. Then majority carriers in the N-region i.e., holes are moving towards base from emitter, reach to base due to repulsion. But, base is lightly doped hence few number of electrons combined with holes that are present in base. And remaining electrons are moving towards collector which already in reverse bias hence large current flows through the load means that high voltage is developed at the output. Hence, in this situation transistor act as amplifier.
See the following link for more details with neat explanation.
Transistor Operating Details
Hope, You got it....