Every explosion needs an ignition source, and within many types of combustion engines, the source is the spark which jumps from the tip of a spark plug.
The plug is a component which forces electricity to jump from one point to another within the combustion chamber, which ignites the air and fuel within the chamber.
Alternative Spark Plug Types
Most spark plugs are the conventional gap spark type, but there are alternative ways to ignite an air/fuel mix.
Instead of having electricity jump a gap, a laser can be shot between a gap to achieve the same igniting effect, but more efficently and over a greater distance.
Plasma, or corona spark plugs, are much more powerful and effective than either traditional or laser spark plugs. It shoots jets of plasma into the combustion chamber, quickly igniting all the fuel. Plasma spark plugs may slightly change engine behavior (since the fuel is completely burned up sooner) and improve fuel efficency.