A system for diesel engines
by which fuel under very high pressure
is distributed through a common line (called common rail
of course) to all the combustion cylinders. The injection
then involves nothing more than the motor management
deciding when and how far to open the injection gates.
With common rail one can achieve high pression diesel injection, which is definitely the stuff that you want in a diesel engine. Competition to common rail is provided by Volkswagen in the form of their proprietary pump injection system.
Common rail was devoloped by the Italian FIAT group (owners of Ferrari and other nice Italian car brands). FIAT sold the design to Bosch, who made it available to other car makers as well.