When I was a child in the 1970s, I was taught that hypotheses become theories upon testing, then upon further testing, theories become laws. Or at least, that's what I remember being taught about the scientific method in elementary school. My memory is not exactly reliable.
Imagine my surprise when I learned that laws and theories are utterly different things and that one did not turn into the other upon further testing.
It seems that I'm not the only one with confusion about this given the number of creationists still making the "evolution is just a theory" argument, so let me try and explain the difference between a scientific fact, a scientific law, and a scientific theory using gravity.
The Fact of Gravity
It bugs me when people say "Isaac Newton discovered gravity" as if human beings did not know that stuff falls before the release of Newton's Philosophiae Naturalis Principia Mathematica. Give our ancestors a little credit. Heck, even many animals know that stuff falls, which is why they are afraid of heights.
If something is confirmed with experiment or observation but doesn't neatly fit into the categories of a law or a theory, it is a scientific fact, although, in the case of gravity, I'm pretty sure it was accepted as fact before Newton's work. The fact of gravity significant to Newton's work is that all masses attract all other masses through gravity, including celestial bodies like planets and their moons. This was very much not obvious to humans prior to Principia Mathematica.
The Law of Gravity
Scientific laws are descriptions that simply describe what reality does, often expressed in the form of an equation.
The Law of Gravity is a simple formula describing the force of gravity between any two objects. It is proportional to the product of the two masses divided by the square of the distance between them. What is significant is that Newton also invented calculus in the same book that explained his law of gravity, and showed how calculus and his equations of motion could be used to accurately predict the motion of the planets around the Sun, or moons about their planets, or a cannonball fired on the surface of the Earth.
A Theory of Gravity
If laws are descriptions that explain what, theories explain the why. If anything, theories are superior to laws because they explain why laws and facts are what we observe. If the Law of Gravity describes the amount of force between any two objects due to gravity, a theory of gravity would explain why the force of gravity between any two objects follows that law, as well as explaining why gravity causes all masses to attract all other masses.
When he published Principia Mathematica, people asked Newton if he had a theory of gravity to go with his law of gravity. The answer he gave was so wrong that physicists don't like to talk about it.
If we take Newton's laws of motion together, along with his explanation for how to use them to describe the motion of physical objects, we can take Principia Mathematica collectively as a theory of motion, but humanity went without a theory of gravity for hundreds of years.
Until Einstein.
General Relativity
Einstein told the world about special relativity but after a decade of hard work, he proved that relativity had something to say about the nature of acceleration, not just velocity.
Newton published Principia Mathematica in 1687. Einstein published Special Relativity in 1905. General Relativity was published in 1915. Two and a quarter centuries after the law of gravity, science finally had a theory to go with the law.
So why do all masses attract all other masses? According to General Relativity, mass distorts the shape of spacetime, which can alter the path of objects passing near said mass in such a way that we interpret as the application of force acting between the two masses. When a comet slingshots around a moon, the spacetime continuum distorts such that the changes in direction and speed made by the comet represent the shortest possible path through the four dimensions of spacetime.
A Theory, Not The Theory
While General Relativity finally gives us a theory of gravity, it's not a very satisfying explanation. Why does the existence of mass change the shape of spacetime? Also, there are hints of problems with the description of gravity provided by General Relativity.
For example, every attempt to reconcile General Relativity with Quantum Mechanics results in mathematical impossibilities, namely infinite values that we know should be finite. On top of that, it is still possible that Dark Matter and Dark Energy are simply errors with how we calculate the effects of gravity.
Theories Don't Become Laws
I think the notion that theories become laws upon further testing is part of the confusion people are expressing when they make the "evolution is just a theory" argument. Laws and theories have both been verified with evidence, or they would not be considered scientific theories or laws.
The Germ Theory of Disease is a fact. It will never "graduate" to become the "germ law of disease." It will always be a theory and a fact (unless directly refuted by evidence). Plate Tectonics will never "graduate" to become a law.
Two Different Definitions of Theory
Of course, scientists have to confuse things by using the word theory in two different ways. When they say "the Theory of Evolution" or "the Germ Theory of Disease," they are talking about theories that have been verified through evidence and are facts.
However, when physicists talk about "string theory," they are actually talking about a hypothesis that has yet to be verified in the way that evolution or the Germ Theory of Disease has been verified. In this case, a theory is an explanation regardless of whether it has been verified with evidence, while in the case of evolution, a theory is an explanation that has been verified with evidence. Personally, I would prefer if people referred to it as the "string hypothesis" for now. Sometimes I wonder if string hypothesis proponents call it a theory out of optimism or hopefulness.