[Kudosdude]

Here is an exploded view of a carburettor.

Very oversimplified;
Carburettors work through suction of fuel via jets.
Injectors work by mechanical pumping of fuel via injectors.

Jet tuning involves changing the size of needles so that the suction causes the correct flow of fuel for the given situation.

Ish. I must point out I'm not really an engine bod so this could no doubt be explained better by someone else.