A Theory is a series of statements about the causal elements for a given effect. In common parlance anything that attempts to describe a cause can be dubbed a theory. In science theory has a far more specific meaning, it is a well substantiated explanation for a series of facts and observations. Theories are the main goal in science and no explanation can achieve a higher "rank."
Common misconceptions about theories
Just a theory
Creationist and Intelligent design proponents often like to describe the Theory of Evolution as just a theory. This relies on equivocating the common usage of the term theory meaning "wild guess" with the scientific term.
The scientific term hypothesis covers what most lay people call a theory. Theories are the single highest level of scientific achievement and nothing is just a theory. Examples include the theory of gravity, plate tectonics, atomic theory, and germ theory.
Theory and Law
Another common misconception is that a theory is the step you go through while on your way to a law of science. Scientific laws and theories are two very different things and one never becomes the other. Scientific laws are factual observations usually derived form mathematical modeling. Theories are the causal explanations behind what creates the laws and observations of science.
Theories can be presently falsified or unfalsified.
Many people think that once a scientific theory has been proven wrong that it is no longer a theory. This is not true, even falsified theories remain theories. For example the phlogiston theory is certainly falsified but is still a theory. Theories come in two varieties: falsified and yet to be falsified. Both are still theories.