short answer: a theory is bigger.

A law is generally a simply-stated principle, like "the laws of physics in one initial frame of reference are precisely the same as in any other" but a theory is an entire model of part of the universe--e.g. special relativity takes the above principle, along with "the speed of light in a vacuum is measured to be the same regardless of velocity of the source or measurer" and from that is deduced the Lorentz equations. From these you could deduce other laws, or properties, such as time dilation, etc.

Actually I said "principle", but usually, a principle is something emergent, such as the laws of thermodynamics, presumably emergent from relativity and quantum mechanics--in chess, the laws are the usual rules of chess, the entire system of rules is the theory, and emergent principles are strategies (like--try to achieve control of the center of the board near the beginning of the game).

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Todd (Bowie, MD, US, North America, Earth, Sol System, Vega region, Local Bubble, Orion arm, Milky Way Galaxy, Local Group, Virgo A Cluster, Virgo supercluster, the universe in which spock is clean shaven)

Quidquid latine dictum sit, altum sonatur.

personal page: http://blog.astrosketches.info