Common rail is where you have a rail with a constant pressure in it. This feeds the injectors constantly.
The injector is then opened with an electric pulse (In the case of a HDi engine it's 80v, little current).

A mechanically pumped engine is a much like a distributor in that it supplies each injector at the right time with the right amount of fuel.

Bare in mine that ANY diesel can run on WELL MADE bio. People who moan about issues running on Bio have been filling up with something a little dodgey.

Also if you want to run someone like pure vegtable oil you will need a Bosch Ve fuel pump and have an Indirectly injected engine.
Even if a direct injection lump has a bosch pump, running it on Veg can gum up the rings.