Modern diesel injectors tend to fall into one of two categories, HEUI and Common Rail. HUEI injectors are found on most the 1994 to 2009 Ford diesels and are both electrically and hydraulically activated. The hydraulic portion is actually operated by high pressure oil from the engine supplied by a separate high pressure oil pump, this makes keeping your oil and filters clean extremely important, as well as the use of quality parts while doing so.
Common rail injectors are electronically activated by piezzo crystals and are supplied extremely high pressure fuel (20 to 30,000 PSI) by a high pressure pump. It goes without saying for either system that clean, quality fuel is a must as well as quality filtration. The injectors are activated by the engine control module based on inputs from various sensors on the engine that tell the computer what the power demands are.