Both are your skin thickening to protect itself from pressure. Same process, different shape — and that shape is why one hurts and one usually does not.
A callus is broad and flat. Pressure spread over a wide area, so the skin thickens evenly. The pad under your big toe, the side of a heel, a guitar player's fingertips.
A corn is small and deep. Pressure focused on one point, so the skin builds a hard core that presses inward like a stone in your shoe. That core is what hurts.
Neither is an infection, neither is contagious, and both come back if the pressure stays.
Usually under a centimetre, with a defined edge and a hard centre you can feel.
Can cover several centimetres with no clear border. It fades into normal skin.
The hard core presses down into the tissue underneath. It feels like standing on a pebble.
It is a protective pad. Most people only notice a callus when it cracks.
Tops and sides of toes, between toes, and the ball of the foot. Shoes that press or rub in one spot.
Heels, the ball of the foot, palms, and fingers. Anywhere that takes repeated broad friction.
A hard corn sits on bony spots. A soft corn forms between toes, where sweat keeps it moist and white.
Dry, thickened, sometimes yellowish, and it may crack at the edges.
Narrow shoes, a seam, a bony prominence, a toe that sits higher than the others, or the way you walk.
Walking, standing, going barefoot, sport, tools, or an instrument.
Padding around it, wider shoes, and paring by a professional for immediate relief.
A callus is doing a job. Treating it only matters if it is thick enough to crack, or is uncomfortable.
Wider toe box, cushioned insoles, a pad that surrounds the corn rather than sitting on it. If the pressure stays, the corn returns however many times it is pared off.
Urea or salicylic acid cream, gentle filing after a shower, and moisturizer. Slowly, not all at once.
Not with a blade, a razor, or a callus shaver. You cannot judge depth by eye, and the two most common results are an infection and a wound that takes weeks to heal.
Medicated corn plasters with a hole in the middle are also worth avoiding if you have any circulation or nerve problem — the acid does not know where to stop.
Do not treat either one at home. Reduced sensation means you will not feel a small injury, and reduced blood flow means it heals badly. A corn or callus in those circumstances is a foot-care appointment, not a drugstore purchase.