A principle is a concept or value that is a guide for behavior or evaluation. In law, it is a rule that has to be, or usually is to be followed, or can be desirably followed, or is an inevitable consequence of something, such as the laws observed in nature or the way that a system is constructed. The principles of such a system are understood by its users as the essential characteristics of the system, or reflecting system's designed purpose, and the effective operation or use of which would be impossible if any one of the principles was to be ignored.
A theory is a contemplative and rational type of abstract or generalizing thinking, or the results of such thinking. Depending on the context, the results might, for example, include generalized explanations of how nature works. The word has its roots in ancient Greek, but in modern use it has taken on several related meanings.
To sum it up, The main difference between Principle and Theory is that the Principle is a rule that has to be followed or is an inevitable consequence of something, such as the laws observed in nature and Theory is a contemplative and rational type of abstract or generalizing thinking, or the results of such thinking.
