All a fuel injector really is is an electronically controlled valve.  The fuel pump sends pressurized fuel through a pipe called the fuel rail to the injectors, which are capable of opening and closing many times per second depending on the position of the throttle. 

When the injector is energized, a large electromagnet opens the valve, which allows the pressurized fuel to spray out through a small nozzle. The nozzle is designed to spray the fuel in a mist that is as fine as possible so that it can burn easily.

The fuel injector's injectors are attached inside the intake manifold because they spray right into the intake valves. The amount of fuel that the injectors spray is dictated by the ECU. (For more information please see my ECU page)