Klinker, also spelled Clinker is a hardened form of ash. Mainly it is made up of silicon dioxide. Removing Klinker can be quite difficult in some stoves. Using something such as Oyster Shell can dramatically soften the klinker and make it much easy to break up and remove.
There are also many varibles in klinker formation such as the type of corn you are burning, the moisture content of the corn and individule burning habits. All of these factors contribute to how much klinker you have and how hard it is to remove.
Klinkers are only formed by burning corn, not wood pellets.