Ignition is the process of providing the energy that is required to initiate a combustion process. When hydrocarbons interact with molecular oxygen to combust, it requires what is known as activation energy to do so - the minimum energy needed to "activate" a chemical reaction.
An example of this would be if a pile of wood in the backyard: there is wood, and there is plenty of oxygen in the air, however there needs to be an initial intense bit of temperature to supply the energy to initiate the chemical chain reaction. In this case the energy originates in the friction of the match along the matchbox's striking surface, which ignites the chain reaction within the match, and can then be used to ignite the fire.
Ignition can be caused by a spark - this what a spark plug does in an internal combustion engine - which is somewhat intuitive to understand. Another way to ignite a chemical reaction is through compression - decreasing the volume of a closed container - which increases the temperature of molecules within the container (by the ideal gas law) and provides the necessary activation energy. Diesel engines rely on this technique for ignition.
Visit UC Davis ChemiWiki for more information on ignition and activation energies.