A callus looks like a mound of dead tissue sitting on the skin. A corn, on the other hand, is like a tiny toy top imbedded in the skin. Also, a corn, unlike a callus, has a hard core. This is why if you press on a corn, it often hurts as the pointed hard core pushes on bone or tissue. Pressing on a callus normally doesn't hurt unless it is inflamed.
Prevention is about the same for both.
Reduce pressure by being sure your shoes and socks fit properly and are in good condition. Keep potential problem areas soft by routinely apply petroleum jelly. And you may want to try the new viscoelastic or Spenco shoe liners. They absorb energy and thus reduce friction. Treating a callus is usually fairly simple.