The IDM (Injector Driver Module) is basically a dumb box that fires the injectors in response to a signal from the PCM telling it which injector to fire, when to fire it, and for how long. It has some basic diagnostic capabilities for detecting short or open circuits, and reports any problems to the PCM via a simple serial communication link.
All in all, they are very well designed devices, except for a vent in the case intended to allow for changes in pressure due to elevation or temperature changes. The vent also lets water in, with no provision to let it back out. This kills the majority of IDM's, and also means that a large percentage of the cores which we get back are simply not rebuildable, which in turns pushes up the price of an IDM so we can cover these losses.