Calluses are thickened areas of skin on the hands and bottom of the feet caused by friction. Corns are calluses on the toes (although more painful is the bunion, a firm, fluid-filled pad, or bursa, on the inside of the joint at the base of the big toe, typically caused by a shoe's rubbing against an abnormal outward projection of this joint; an abnormality which may also be inherited).
To treat a developing or exisiting corn, massage it with a lotion that contains lanolin or urea to soften it (but, do not use abrasives or attempt to pumice hard corns--it will only worsen the condition).
To treat calluses only (not corns) soak your feet in warm water with Epsom salt for about 10 minutes, then use a pumice stone or callus file to rub the top layers off the callus.
Also, to minimize discomfort of corns and/or calluses:
1. Wear shoes that fit properly and comfortably.
2. Shoe inserts
3. Toe bandages (prevents friction)
4. See a foot specialist if your condition worsens or if you have a pre-existing condition (such as diabetes, etc.) that may impact any treatment.