The starter is an electric motor that spins a small gear; that small gear then spins the flywheel making the engine turn over. If anything thing goes wrong in this process, it can cause the engine not to start at all. There are several telltale signs that there is a problem with either the starter or flywheel that is preventing your car from starting or causing starting issues.
Free spinning is when, for some reason, the starter gear is just spinning without contacting anything. The sound is similar to that of a small drill spinning. When this happens there can be an issue with either the flywheel or the starter. In the case of the flywheel, the teeth might be sheared off in one area, or off the entire flywheel. In the case of the starter, the gear could be loose or the starter engaging motor--the one that pushes the gear onto the flywheel--might be malfunctioning.
A grinding sound can occur when one of two things happens. The most common problem is that the starter gear has broken and is making noise. The less common of the two is that the teeth of the flywheel have been worn down. This causes the starter gear to not engage properly and grind against the flywheel.
When a starter is "over-starting" it is actually not disengaging from the flywheel properly. This causes the flywheel to continue to spin the starter, creating a buzzing type of sound for as long as the starter stays engaged. This is a relatively common problem in early 1990s Nissans. This noise can sometimes occur only for a fraction of a second, but it can also occur the entire time the car is running.
When you hear absolutely no noise from the vehicle, this typically means that the starter motor has failed to turn the engine over. A starter motor can sometimes have a "dead spot," which means there is one area of the starter that is electronically bad. This problem causes an occasional non-starting condition. You can sometimes get beyond the "dead spot" by lightly tapping the starter with a hammer, which can help the vehicle to start. This same problem can be caused by a dead battery or a loose wire--make certain to check these items as well.