In science a scientific law is a portion of a Scientific Theory which predicts the actions of objects under most circumstances. Laws almost always have exceptions. Examples of this from physics are Newton's second law (F=ma) which is true only at speeds well below the speed of light (see Theory of relativity, Hooke's Law of springs (F=-kx) which is true for only "Hookean" springs (examples of non-Hookean springs are screen door springs and leaf springs used for suspension in heavy trucks), and Ohm's Law (I=V/R), which fails under extreme temperature, being unable to predict superconductors and increase in resistance due to heat. Another example is Kepler's law of planetary movement, which predicts the movement of planets very well, but is unable to account for all of Mercury's orbit, which is explained by the theory of General Relativity.
There is much confusion in the general public over the difference between a Theory and a Law in science. A theory is an overarching explanation for a set of observed phenomena. A law, is a tool use for predicting behavior (see the examples from physics above). Scientific laws are less useful than theories, as they do not explain the world, they simply predict events. Theories are not promoted to laws when they accumulate enough evidence. Instead, hypothesis become parts of theories when there is enough evidence to show that the explanation is the best for the observed phenomena. This is different from conventional use, in which theories are assertions backed by scant evidence and laws are statutes enacted by a government.