When wearing a tight shoe, you may feel that a particular part of the shoe is being repeatedly rubbed and stimulated, and later the surface of the skin becomes hard and thick. A corn is the center of the thick part of the skin layer (keratinocytes) and it becomes hard. When pressure is put on that point, pain will be felt in the nerves. It usually forms in the joints of the toes, and as long as you don’t have the center of the corn removed, it would not be cured.
Caluses are much the same as corns. They form by repeated pressure or stimulation, but the difference is that calluses do not have the center made of keratinocytes and forms a thick and hard skin in a wider area. If this part stimulates the nerves then pain will occur. It usually forms in the sole of the foot or on top of the bones. As long as it stimulates the same point, even after shaving the hardened area of the skin, it will form again.
When wearing new shoes or new socks, before you get used to them, those might cause friction in part of the foot which usually does not get rubbed and this may cause water blisters. You can use a protection pad in advance and use it to prevent the part which is likely to cause abrasion. Or if it happens that you have a water blister, then make a hole with a needle and remove the fluid inside, keep it clean and take measures to prevent infection. After this you can apply the protection pad.