In everyday use, theory means a guess or a hunch, something that maybe needs proof. In science, a theory is not a guess, not a hunch. It's a well-substantiated, well-supported, well-documented explanation for our observations. It ties together all the facts about something, providing an explanation that fits all the observations and can be used to make predictions. In science, theory is the ultimate goal, the explanation. It's as close to proven as anything in science can be.
Some people think that in science, you have a theory, and once it's proven, it becomes a law. That's not how it works. In science, we collect facts, or observations, we use laws to describe them, and a theory to explain them. You don't promote a theory to a law by proving it. A theory never becomes a law.
This bears repeating. A theory never becomes a law. In fact, if there was a hierarchy of science, theories would be higher than laws. There is nothing higher, or better, than a theory. Laws describe things, theories explain them. An example will help you to understand this. There's a law of gravity, which is the description of gravity. It basically says that if you let go of something it'll fall. It doesn't say why. Then there's the theory of gravity, which is an attempt to explain why. Actually, Newton's Theory of Gravity did a pretty good job, but Einstein's Theory of Relativity does a better job of explaining it. These explanations are called theories, and will always be theories. They can't be changed into laws, because laws are different things. Laws describe, and theories explain.
Just because it's called a theory of gravity, doesn't mean that it's just a guess. It's been tested. All our observations are supported by it, as well as its predictions that we've tested. Also, gravity is real! You can observe it for yourself. Just because it's real doesn't mean that the explanation is a law. The explanation, in scientific terms, is called a theory.
