Spark plugs eventually wear out and require periodic replacement. The spark plugs can also be very useful in diagnosing how an engine is running by removing them and inspecting the tips. Be sure to keep them in order so you know which spark plug went in which cylinder!

This video covers "everything you wanted to know" about spark plugs.

The spark plug broke off when trying to remove it

It is possible for old, corroded spark plugs to break when trying to take them out. This topic covers that situation.