Theory vs Law
Scientific Hypotheses, Theories, Laws, and Models
The United States National Academy of Sciences describes a theory as follows:
A scientific theory is a scientific explanation so well established that no new evidence is likely to alter it. In everyday language, a theory means a hunch or speculation. This is not so in science. In science, the word theory refers to a comprehensive explanation of an important feature of nature supported by facts gathered over time. Theories also allow scientists to make predictions about as yet unobserved phenomena.
A scientific theory is a well-substantiated explanation of some aspect of the natural world based on a body of facts that have been repeatedly confirmed through observation and experiment. Such fact-supported theories are not “guesses” but reliable accounts of the real world. The theory of biological evolution is more than “just a theory.” It is as factual an explanation of the universe as the atomic theory of matter or the germ theory of disease.
The primary advantage enjoyed by this definition is that it firmly marks things termed theories as being well supported by evidence. This would be a disadvantage in interpreting real discourse between scientists who often use the word theory to describe untested but intricate hypotheses, in addition to repeatedly confirmed models. However, in an educational or mass media setting it is almost certain that everything of the form
theory is an extremely well supported and well tested theory. This causes the theory/non-theory distinction to much more closely follow the distinctions useful for consumers of science (e.g. should I believe something or not?)
Scientific laws are similar to scientific theories in that they are principles that can be used to predict the behavior of the natural world. Both scientific laws and scientific theories are typically well-supported by observations and/or experimental evidence. Scientific laws usually refer to rules for how nature will behave under certain conditions, while scientific theories are more overarching explanations of how nature works and why it exhibits certain characteristics.
Around the year 1800, Jacques Charles and other scientists were working with gases to, among other reasons, improve the design of the hot air balloon. These scientists found, after many, many tests, that patterns and regularities existed in the behavior of gas. If the temperature of the gas increased, the volume of the gas increased. This is known as a natural law. Natural laws describe the patterns we see in large amounts of data but do not describe why the patterns exist.
A common misconception is that scientific theories are rudimentary ideas that will eventually graduate into scientific laws when enough data and evidence has been accumulated. A theory does not change into a scientific law with the accumulation of new or better evidence. Remember, theories are explanations while laws are patterns we see in large amounts of data, frequently written as an equation. A theory will always remain a theory, a law will always remain a law.
From http://www.ck12.org/book/Introductory-Chemistry/r1/section/1.1/Introduction-to-Chemistry-and-The-Nature-of-Science-%253A%253Aof%253A%253A-Introductory-Chemistry/