Electrical problem
1995 Mercury Tracer 4 cyl Front Wheel Drive Automatic 125,00 miles

i drove my car earlier today and then shut it off and it would not start again. it did appear to have a lot of corrosion on the negative battery cable, and the attached sensor. previosly i would have to turn the key several times in order to get it to start. now when the key is put in the ignition all of the dummy lights come on and stay on even if the key is turned to the off position. when the key is turned to the on position the car does not turn on and it doesnt even make sounds as if it is trying to start. when i tried jump starting the car nothing happened either. i am very confused by the dummy lights being stuck on.

My first question is what is the sensor called that is attached to the ground cable?

my second question is, what might be causing this problem, the battery cable? the starter? possibly the ignition? or something else.