>I've found in my general experiences that many people don't know what a scientific theory is. They think it means a guess.
Well they are correct.
Well they are correct.
Law: A descriptive generalization about how some aspect of the natural world behaves under stated circumstances.
Hypothesis: A testable statement about the natural world that can be used to build more complex inferences and explanations.
Theory: In science, a well-substantiated explanation of some aspect of the natural world that can incorporate facts, laws, inferences, and tested hypotheses.
(Source: http://arstechnica.com/science/2006/08/5164/)
Edit:
How it works...
"Massive objects are attracted to each other by the force of gravity." == Scientific Fact.
"Objects in motion remain in motion. For every force there is an equal an opposite reaction. Force is mass times acceleration." == Scientific Law (Newton's Laws of Motion).
"The bending of space under the influence of gravity can cause light to curve around massive objects." == Scientific Hypothesis (Verified by Eddington's 1919 solar eclipse observations).
All of the above (and a whole lot more) put together == General Theory of Relativity.
...that can be wrong and often are replaced by better theories as more tests and better explanations came out.
Science is based in doubt. If you are too sure of a scientific theory, you are missing what science really is about and what you need is religion.
But a Theory is not a guess. It is a framework built from individual pieces, some of which may be hypotheses (effectively guesses) that have been tested and found to hold within the larger structure of that framework.
Nothing is known with absolute certainty. What is the point is referring to everything as a "guess" simply because it isn't known for certain? Context is important here.
It was a very good guess though.
Newton's LAWS of Motion are not a THEORY. They are a descriptive generalization about how gravitation behaves under stated circumstances. Those circumstances being slow speeds and large masses. In those circumstances, they are not wrong. They are a generalization. The THEORY of General Relativity contains Newton's LAWS of motion as a part of the overall framework.
It's the same with almost any natural science theory. Theories try to be very good aproximations of the truth, until a better guess comes out.
I'm sure you're aware, there's a bit more to it than that.