Distributors are gear driven ignition systems, using a gear attached to an engine's timing system to accurately distribute the electrical spark down a plug wire and to the spark plug so combustion happens at precisely the right time (usually TDC, or top dead center, when the combustion chamber is at maximum compression).
When the distributor fails, an engine's timing will likely be off causing backfires or poor performance. In some cases the engine may not run at all!
The difficulty of replacing a distributor will depend greatly on the vehicle in which the repair is being undertaken.
We stock and sell a wide variety of quality distributors from these manufacturers: