Port injection has one injector per cylinder, and the injectors are located below the throttle. This means that the injectors see full manifold vacuum, and the fuel pressure must be 'vacuum referenced' (the fuel pressure regulator is connected to a manifold source of vacuum, and the fuel pressure falls with increasing vacuum so the the pressure across the injector is constant).
Port injection can be simultaneous, batch, and sequential. Simultaneous means they all fire at once. Batch means some squirt (usually half of them) then the others squirt. Sequential means that each cylinder's injector fires at the same spot in its intake-compression-power-exhaust stroke cycle.
Your MicroSquirt® controller will do batch or simultaneous injection for up to 8 cylinders, and sequential injection for one or two cylinders.
Port injection injectors are typically (but not always) high impedance (> ~10 Ohms) in factory installations.
Throttle body injection squirts above the throttle plates, and generally has fewer injectors than cylinders (but not always). Throttle body injector units often have the throttle plates, injectors, fuel pressure regulator, and TPS altogether in one unit, making for a simple installation.
Throttle Body injection injectors are typically (but not always) low impedance (< ~3 Ohms) in factory installations.
Your MicroSquirt® controller operates by grounding the injectors. This means that you must wire one pin of the injector to have 12V whenever your MicroSquirt® controller does, and the other goes to the Ampseal pins #9 or #10 which ground the injector(s) so that current can flow and the injector will squirt. Note that the injectors must never have 12V when your MicroSquirt® controller does not, or you could hydraulically lock your engine, causing a great deal of damage.