What is the difference between a design and utility patent? How?

A utility patent covers a totally new invention; a design patent covers a new form of an existing invention such as a car bumper.

A design patent protects the ornamental design of a physical item. Think of the shape of items - they may be protected by design patent if the design is distinctive from prior designs. Design patents must, like utility patents be new and not obvious. Utility patent as protect objects and what they do, and often how they are made - if the invention is new, useful and not obvious. There are also plant patents - a special category often used to protect roses, among other plants.