Causes: Pressure and friction also caused by mechanical stress as above.

Appearance: In this case, the hard skin is an inverted cone and is pushed into the skin causing discomfort.

There are various types of corns: 
Hard corns, usually found on the soles of the feet or the tips and front of the toes
Soft corns, usually found between the toes
Seed corns, which are smaller and found on the soles.