Others have answered this question pretty well. The
answer is that some cars far the fuel all the time,
others fire all injectors, others fire the injectos
in groups, and still others fire the injectors one at
a time at the correct time.

Each is a trade-off in terms of complexity versus
efficiency. At high rpms, there is almost no
difference between the continuous and the sequential
fire systems. At low RPMS, the fuel hangs around in
the intake port before the intake valve opens and some
of it will coalesce into bigger drops that don't
burn as well. This only really affects emissions, not
power or fuel efficiency.

The short answer is that, for a 4 cylinder motor,
if the injectors all fire at once, each pulse is timed
to only fire 1/4th the necessary fuel.

A good couple of web sites are:
www.sdsefi.com
www.diy-efi.org
chris