The bipolar transistor can be described as 3-electrode, enghancement mode, controlled current source. The electrodes are the control-pin ´Base´, the high-ohmic ´Collector´ and the low-ohmic ´Emitter´.
A small vaued Base current can steer a much larger Collector/Emitter current.
The ratio of Collector current to Base current is called DC-current gain or hfe in Datasheets. Typically one rather works with steering voltages instead. The graph of Ic over Ube has a exponential curvature with a steep bent at around 600mV. If Ube is lower the transistor is cut off.
It opens up when the Ube surpasses the 600mV treshold. A collector current flows and the device may be used as amplifying device or as switch.
Due to the required Base-biasing, the circuits need to be designed accordingly and/or additional devices are required for biasing purposes.
NPN and PNP types differ in polarity. Ube and Uce (the voltages measured from the base resp. the collector to the emitter) are positive for NPNs and negative for PNPs.