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Simply put, the fuel injector in your engine is in charge of putting fuel into the combustion chamber. Each cylinder has its own fuel injector. The fuel moves from the tank through the fuel lines and into the injector before it gets dispersed.
These days most diesel cars and quite a few petrol vehicles use a direct injector, which delivers the fuel in precise bursts as it is pushed through a nozzle under high pressure.

Direct vs. Indirect fuel injection

In cars with an indirect fuel injection system, the fuel goes through an injector into the inlet manifold or a so-called pre-combustion chamber, where it is mixed with air and ignited. That in turn forces the fuel into the combustion chamber. 
Direct fuel injection brings the fuel, as the name implies, directly into the chamber. This happens under high pressure and once the fuel meets the incoming air stream in the cylinder, self-ignited combustion occurs, moving the pistons. 
Most fuel injectors are controlled electronically these days with an ECU (Electronic Control Unit) corresponding with the injector's solenoid plunger. The unit monitors and balances speed, torque, air temperature, etc., making sure that the administration of the fuel-air mixture is perfectly timed. This makes the car more powerful, fuel consumption more economical, and most importantly it reduces emissions. Altogether, electronic fuel injection systems are more efficient and more reliable. 

Why does an injector fail?

Injectors have a tough job considering that they constantly deliver high-pressured fuel through tiny nozzles. Needless to say, this can cause fuel injector problems over time. 
There are a few reasons why the fuel injector can fail. These are the most common ones:

Solenoid failure - The solenoid can wear out over time or malfunction. This causes problems because the nozzle won’t know when to open and thus fuel cannot be injected. 

Spring malfunction - The return spring in the injector can malfunction if there is damage to other engine components. Especially at higher RPM, this can cause the fuel system to misfire. 

Clogged nozzle - By far the most common cause of a failing fuel injector is a clogged nozzle. This usually happens over time due to the use of low-quality fuel or a dirty fuel filter. To prevent this from happening you should only buy high-quality fuel and change the filter regularly.