Tell tale signs of a failing distributor include (but are not limited to) a weak or irregular idle, wildly varying RPM's, stalling, back-firing, and difficulty starting the car. All this is because your ignition distributor is the governing body in charge of regulating your engine's timing by sending the spark to your plugs according to the correct firing order. If there is a failure in the accuracy of this process, your spark plugs won't detonate the air/fuel mixture inside the combustion chamber and your vehicle will suffer from a severe lack of power. And it isn't always easy to tell what parts of the distributor are at fault, either. The brushes on the contact points may be frayed, the distributor cap could be cracked or corroded, or the rotor might not be turning properly. Not only can this be an inconvenience to say the least, but a faulty ignition distributor will soon definitely fail and could leave you stranded if your car shuts off and can't restart.
For the better part of the 20th century, automakers had settled on mechanically timed ignition as the most efficient and reliable method of delivering an accurately timed spark in a fixed order at varying frequency. If that sounded like gibberish, here's a simple explanation: The ignition distributor is driven by a small gear at the bottom of its drive shaft that rotates along with the engine camshaft. That spins a small cam inside the distributor that opens and closes the contacts on breaker points. When the points are closed, power from the battery is stored in the ignition coil. When the points are opened, that power is transferred from the coil to the top of the distributor cap where it is sent to a small rotor inside. That rotor also turns along with the same rotation of the camshaft, and at precise moments the charged rotor comes in contact with leads in the distributor cap that send the current through ignition wires and to the spark plugs. This process repeats itself literally thousands of times per minute. Due to the complexity of this process and the availability of microprocessors starting in the 1970's, mechanical timing was superseded by electronic ignition. Hall sensors replaced breaker points, and took orders from a computer that told the distributor when to ignite and which cylinder to fire. By the end of the 20th century, programmable ignition control modules were doing all the work and distributors were used no longer.
Replacing your distributor is usually an easy task, depending on the location and type of distributor. They are almost always located at the top of the engine near the back of the block and driven by a gear on the very end of the camshaft. Some styles are belt driven, and will likely be located up front. Just follow your spark plug wires all the way back to it.