Bipolar transistors are amplifying devices and can also be used as switches. There are two types: NPN and PNP. See the picture below for a typical circuit.

A bipolar transistor has three terminals: Base, Collector and Emitter. In case of a NPN transistor, a small current flows from B to E (I_B) causes a larger current flow from C to E (I_C). The ratio I_C/I_B is called the current gain, symbol h_FE.

Inside a transistor, there's a diode between B and E and between B and C, so V_BE,max and V_BC,max are about 0.6V to 0.7V.

Let's assume for example RB = 1M, RL = 1k, VS = 9V, h_FE = 300 and V_BE = 0.6V.

The voltage across RB will be VS-V_BE=8.4V, so I_B=8.4/1M=8.4μA. I_C=I_B∙h_FE=8.4μA∙300=2.52mA. So the voltage across RL will be 2.52V.