its a method of injecting fuel into the engines through a nozzle so that the flow of petrol is controlled using chips to increase fuel efficiency...it replaces carburetors of conventional petrol engines...
note:deisel engines have never used carburetors and are conventionally fuel injected from yore
direct injection is squirting fuel right into the combustion chamber
most gas car fuel injection squirts into the intake ports out side the combustion chamber before the air intake valve
so fuel can pool and must be sucked in with the air intake
direct injection is more efficient but requires an injector that can stand hi-temps
and pressures inside the combustion chamber so costs far more to make and the fuel injection must be timed and amount controlled far more requiring greater computer control also adding to costs
Direct injection also injects the fuel during the combustion stroke rather than during the intake stroke, as in conventional fuel injected engines, eliminating pre-ignition problems that high-compression engines are prone to.
I think Ray's post sums it up best, if you're asking about gasoline (petrol) direct injection. Note that most modern diesels use a direct injection strategy.