A Law of physics is a mathematical relationship between measurable quantities that describe the physical state and properties of bodies. This is a fundamental concept in physics.
Collectively, the laws of physics are those physical theories which have been widely published and tested, and are considered by the scientific community in general to be valid. They also tend to be very general, basic theories: instead of having a large list of laws governing many different phenomena in different circumstances, special cases are arrived at through a generalization of basic ideas. Well-known laws of physics include Einstein's Theory of General Relativity, Newton's Laws of Motion, Maxwell's Equations for Electricity and Magnetism, and the theory of Quantum Mechanics.
Interestingly, these so-called "laws" can essentially be viewed as a series of approximations: well-established physical laws are found to be invalid in some special cases, and the new theory created to explain these discrepencies can be said to have generalized the original, rather than superseded it. One well-known example is that of Newton's law of gravity: while it described the world accurately in most normal circumstances, such as the movement of the planets around the sun, it was found to be inaccurate when applied to very large masses or very high velocities. Einstein developed the theory of general relativity, which accurately handled gravitational interactions both those extreme conditions and in the range occupied by Newton's law. However, Newton's formula for gravity is still used in most circumstances, as an easier-to-calculate approximation of gravitational interaction. The same phenomena can be observed when comparing Maxwell's Equations with the theory of quantum electrodynamics, and in other cases.
See History of Physics.