Within every fuel injected engine are a number of fuel injectors. Some engines have just one, but many modern engines have one injector for every cylinder. The fuel injector itself is a spray nozzle which if fed fuel by the fuel rails. The amount of fuel sprayed is determined by the ECU.
Unlike a carburetor, a fuel injector can release fuel at a specific time, and a specific amount, while a carburetor only releases a steady flow based on the vacuum pressure in the intake manifold and the amount allowed through the adjustment pin's current position.
The less distance between the location of the fuel injector and the combustion chamber, the less surface area the fuel is likely to touch, which means more of the fuel actually gets to the chamber, and this leads to a more efficent engine.
The fuel injector system must be designed properly for each different engine due to differences in timing, and can sometimes be further improved with an ECU tune.
Most fuel injection systems are electronic, but some early versions were mechanical.