A scientific theory is a plausible and consistent explanation for observable phenomena. While a scientific law describes something that always happens in the same way, a scientific theory explains what causes it to do so.
For example, doctors have long known that being near a person who has a communicable disease exposes you to a risk of catching it (see quarantine). It was a "law" of contagion that staying far away protected you from catching the disease, even before Pasteur's germ theory of disease provided the modern scientific explanation (the "theory").
A successful theory is most consistent with the actual behavior of the universe. When a phenomenon is discovered that is inconsistent with the prevailing theory, the theory is either revised or thrown out entirely. An example of this is Newtonian mechanics--while Newton's 3 laws of motion provide extremely accurate descriptions/predictions of the behavior of medium-sized objects at low speeds, they are inconsistent with the behavior of very large, very fast, or very small objects. Newtonian mechanics has been shown to be a limiting case of the theory of relativity (in the limit where velocity goes to 0) and quantum mechanics (in the limit where Planck's constant goes to 0, essentially the limit of large objects). Usually a theory which is inconsistent with some experimental evidence is not completely falsified; it is instead shown to be a simplified version of a more complete theory.
A theory is more substantial than a conjecture or hypothesis, but may not be verified as consistent with empirical data.  Controversially, some scientists have declared that causes other than natural, observable ones should not be examined - and that theories involving such causes are "not scientific", e.g., behavioral psychology. This limitation and its implications for the philosophy of science plays a large role in the origins debate.
For many philosophers of science, a theory must be falsifiable to be considered scientific. This means that there must be some way to do experiments that could counter the theory's predictions, thus disproving the current theory. 
Generally accepted scientific theories have survived over time, have evolved when appropriate, and modified toward consistency with newly discovered data, have not been shown to be false and can make predictions about natural phenomena.  Scientific theories can sometimes be used as the basis for industrial and technical developments.
A scientific theory should have to have strong experimental support or accepted by the scientific community. However, some scientists mistakenly refer to untested theories and competing theories. Theories are generally extremely well-confirmed, such as conservation of energy. Some more speculative 'theories' are more accurately labelled 'hypothesis' within the scientific community, such as String Theory.
Some evolutionists frequently claim that the word "theory" means very different things to scientists and non-scientists. For example, a PBS TV show says: 
- When we use the word "theory" in everyday life, we usually mean an idea or a guess, but the word has a much different meaning in science. This video examines the vocabulary essential for understanding the nature of science and evolution and illustrates how evolution is a powerful, well-supported scientific explanation for the relatedness of all life.
The American Museum of Natural History exhibit on Darwin says: 
- In everyday use, the word "theory" often means an untested hunch, or a guess without supporting evidence. But for scientists, a theory has nearly the opposite meaning. A theory is a well-substantiated explanation of an aspect of the natural world that can incorporate laws, hypotheses and facts.
In fact, it is hard to find anyone who makes this distinction, outside those who are promoting the theory of evolution to the general public.
An example of the term 'theory' in scientific literature is in the field of speculative physics, which proposes something commonly deemed 'String Theory'. At it's most basic level, string theory states that the entirety of the universe is made up of tiny, multi-dimensional strings. However, this proposal cannot currently be verified or refuted by modern science - that is, scientists cannot determine if this is right or wrong. Since String Theory makes no verifiable predictions about the universe, either, scientists cannot say that String Theory fits any facts we currently know. Therefore, String Theory is not the sort of scientific theory which is evidenced by facts and must make correct, verifiable predictions about the future. 
Common scientific theories
- Cell Theory - how our bodies are made up of cells. 
- Gravitational Theory - the explanation for how matter attracts other matter. 
- Atomic Theory - the current understanding of the basic makeup of living and nonliving things. 
- Possibly the Theory of Evolution. While this is the most popular term for it, science philosopher Karl Popper asserted that it could be considered a metaphysical research program depending on the definition being used,  and other science philosophers have called it a scientific law.
Surprisingly, modern methods of scientific inquiry were not generally accepted until the late sixteenth and early seventeenth centuries. Thus in terms of human history, scientific inquiry is in its infancy. What Is Scientific Inquiry? - Arizona Collaborative for Excellence in Preparation of Teachers (ACEPT)
Notes and references
- For example, scientists may refer to phlogiston theory or ether theory, which originally sounded plausible even though empirical verification was lacking. As a result, these theories are no longer accepted by scientists.
- "A theory not only explains known facts; it also allows scientists to make predictions of what they should observe if a theory is true. Scientific theories are testable. New evidence should be compatible with a theory. If it isn't, the theory is refined or rejected." 
-  used the word "theory" 70 times without any suggestion that ideas must be well-substantiated before they qualify as a theory. Physicist Lawrence Krauss used the term "string theory" even though the theory has "yet to have any real successes in explaining or predicting anything measurable".  A month later, he argued that it should really be called the "string enterprise" because using the word "theory" causes problems with intelligent design advocates and those who say that evolution is "just a theory". 
- The science philosopher Karl Popper famously called Darwinian evolution a "metaphysical research program".