Early in my Law & Economics course, taught by Don Boudreaux (professor and former chair of the economics department at GMU, blogger at Cafe Hayek) he made a controversial point about the nature of law. While most of us associate "law" with legislation, handed down by Congress or other governing bodies, Dr. Boudreaux argues that we should see law as an emergent phenomenon, developed by the public's expectations for social behavior. For the curious, a similar lecture he gave can be found here.
To borrow his example: if I set down my books at a table in the cafeteria and go to get food, I have a reasonable expectation that when I return, no one will have taken the seat where those books were placed. It's generally accepted that a book (or coat, or other personal item) can act as a stand-in for a person's presence in reserving a space. Someone who took that spot would be breaking a law, even though that standard has never been written down or codified anywhere.
Sometimes law and legislation can be synonymous (e.g. people understand murder is wrong, and it also carries criminal penalties) and in other cases they are contradictory (e.g. speed limits that no one follows, or underage drinking at college that is illegal but generally tolerated). Legislation on the books has no power except to the degree that people will accept it, or the powers of government are willing to use force in order to apply it.
Generally, this makes a lot of sense to me: the Federal Register, which added over 75,000 pages in 2010 that probably less than a dozen people have actually read, probably plays less of a role in public daily life than seat-saving etiquette for restaurants, even though each part of the Register could (theoretically) be enforced with the full power of the United States Federal Government.
My only complaint is a semantic one. This conception of law subsumes and replaces the role of customs, norms, and social mores; those become redundant terms given this definition of "law." This is not necessarily a bad thing - it may even be good, if as Boudreaux argues, we should reframe the public understanding of law as being opposed to legislation - but the loss of precision from duplicating "norms" and "mores" bothers me. (Maybe my attachment to those terms is just the sociology minor talking.)
If I were starting this issue afresh, I'd propose a definition of law as the intersection of legislation and publicly accepted norms. This is much narrower (it includes the prohibition on theft and murder, but excludes unwritten social rules) and arguably has greater analytic power, because it creates a new category for what is both expected by society and enforced by the powers that be.
Of course this issue isn't being started afresh, and any new definition of law has to battle a centuries old Anglo-American tradition of treating legislation as identical to law. Changing that entrenched belief will be a challenge, no matter what theoretical or practical merits a new conception of law might bring.