An idea is a thought defines the general scope of a project or a collection of thoughts created with a specific purpose in mind. A theory is an assumption or presupposition about something. Theories are also created with a specific purpose in mind.
Ideas are often called upon in order to create a plan of action. This plan can be as simple as brainstorming a birthday party or creating a master plan for a marina. Ideas are the beginnings of action. Theories are more passive, used to explain rather than act. They are used to explain cause-and-effect relationships. Theories are used as the basis for research and ideas. By attempting to explain the relationship between two things, you can begin to fix the problem.
Ideas are proved by putting them to action. If the project that grew out of the idea is successful, then the idea is proved correct. Theories, however, must be proved in a different way. For example, a theory that married people are happy is tested by asking married couples if they are in fact happy. The theory is proved if a majority of couples answer the questions positively.
Once an idea is proved to work, it is reused when needed. However, ideas are often created with a certain purpose in mind. New ones are required when a new purpose arises. Theories become fact after they are tested several times and found to be true. These facts are recycled often and used as needed.