The crucial difference is that most existing gas engines have some form of port fuel injection - fuel is sprayed into the airflow outside of the combustion chamber, before the intake valves close. (Even older systems like my 1986 Plymouth had throttle body injection, which basically replaces a carburetor.) Gasoline direct injection (GDI) is like diesel fuel injection - the fuel is sprayed directly inside the combustion chamber, near the sparkplug, after the intake valves have closed. It happens to be sprayed at very high pressure, but the main important feature is being sprayed inside the chamber instead of into the intake port outside of the chamber.
I think gasoline direct injection is going to revolutionize cars and light trucks and will greatly reduce the attractiveness of diesel for lighter applications. GDI lets an engine run similarly to a diesel at light loads, with very lean mixtures but without detonation that you'd get running those lean mixtures with port injection. It all has to do with how fuel is mixed inside the combustion chamber. At high loads it runs pretty much like a port-injection gas engine, although still a bit more efficient because of better spray pattern and mixing, and no fuel clinging to the cylinder walls and getting wasted. Also it should cost less than a comparable diesel engine, and it doesn't require a turbocharger for good performance. (Diesels don't absolutely require a turbo, but to meet on-road power, emissions and weight requirements, they pretty much practically require a turbo.) On top of all this most engines converted to GDI gain a lot of horsepower compared to their port-injected relative. The GDI engines are starting to appear, for instance in the newest Cadillac sedan, and are probably going to be common in just a few years.
