A corn and a bunion are very different problems. They occur on different areas of your feet. One is a skin problem and one is a bone problem.
A corn is simply hard skin that forms over a bone bump on the top of the toes. A soft corn can form between the toes. This is only skin that hardens to protect a bone deformity such as a hammertoe or bone spur.
A bunion is a bump of actual bone that forms behind the big toe. It is a bone deformity, not a skin problem. The bunion forms when the first metatarsal bone moves outward and causes the bunion bone bump. The big toe will often move toward the second toe as a result.