Calluses and corns occur when your skin thickens in order to protect itself from some sort of outside irratant like friction for example. Most people only need treatment for a callus or corn if they are causing discomfort. Most of the time, eliminating the source of irratation would eventually allow the callus or corn to heal on it's own. They are both characterized by thick, rough, hardened and/or raised skin.
Calluses and corns are NOT the same, however. Corns are smaller than calluses, develop on non-weight bearing areas and are usually painful when touched. Calluses tend to be larger in size and usually don't have any associated pain.
You only need to see a doctor about a corn or callus if they become painful and/or infected. Self-treating either affliction is quite manageable and easy with household items.
Causes
Calluses and corns are both caused by a variety of different things. However, the root cause will always be repeated friction and/or pressure on the affected area. Specific causes include but are not limited to: wearing shoes that are too tight, not wearing socks, playing an instrument and lifting weights. Bunions and hammertoe can develop corns and calluses easily.
First Aid & Prevention
Specific ways you can treat a corn or callus can include wearing shoes with lots of room and avoiding friction and pressure as much as possible. Realistically, you don't need to do much other than eliminating the cause of the corn/callus in the first place.