MPFI: Multi-Port is one injector per cylinder, like the 2.8s and most other somewhat performance-oriented engines, PFI is the same thing AFAIK. good for performance, as each cylinder can have a different fuel requirement based on conditions. also know as SMPI, SEFI, SPFI; S stands for Sequential.
TBI: Throttle Body Injection, one injector (some hondas used two, known as DPI, or Dual Point Injection) spraying into either the throttle body or intake manifold. cheap and easy.
TPI: Tuned Port Injection, you see these on the high performance LT1/LS1 5.7s, as well as the later model performance V8s. the only real difference i believe between regular PFI and TPI is the long intake runners, which allow for the air to arrive at the combustion chamber at more "convenient" times, which results in a bit more power, and a flatter powerband.
GM also came out with something known as CPFI, or Central Port Fuel Injection, which was kind of a hybrid of a MPFI and TBI system, it used the single injector, but it had lines to each individual cylinder. this is not to be confused with CFI, or Crossfire Fuel Injection (aka Cease-Fire Injection), which was used on early 80's z28s (81-83?) this was a form of TBI, but using two diagonally mounted Throttle Bodies, which i guess was nice while it lasted. they had a habit of failing and then you were screwed, as i've been told not many people bothered learning how to fix them, and were relatively complex?
(sorry for the rant on CFI, i used to have an 83 Z28
)
there are other iterations from different companies, things like Direct Fuel Injection, where the fuel is injected straight into the combustion chambers; and other names for the same thing like SEFI, SMPI, CFI (Central Fuel Injection, which is basically the same as TBI)
hope that helps, got this stuff with about half hour of playing on google
