Tenth Amendment:
The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people
The bill of rights lines out specific areas that the states and fed cant (shouldn't be able to) mess with. Anything else is up to the states; I.e., gay marriage, land use, contract law, etc.
I believe that the mis-application of the tenth amendment is the root cause of our current woes as a country. We are (or were meant to be) a union of states much like the EU, and not a homogeneous country. The constitution was designed to outline some basic inalienable rights that were self evident to the people at the time. These rights were not "granted" by the government, They were endowed to you by your creator. Everything outside of the very limited power the granted to the federal government is reserved to the power of the states.