A hill is a high ridge or mound of land, not as tall as a mountain. A house on a hill has a great view of the surrounding countryside, and very little chance of being flooded.
You can hike or drive up a hill — a gentle upward slope, in either case. An accidental or deliberate mound of stuff is another kind of hill, like a hill of snow in your yard, or a hill of trash at the dump. What's considered a hill in mountainous India — a height of up to 10,000 feet — is absolutely a mountain in Great Britain, where a hill is defined as an elevation of less than 2,000 feet.