Executive Orders???

As Barack Obama has become our new president, the news tells us that he will immediately, upon innaugaration, use "executive orders" to enact his policy priorities. (e.g., more funding for embryonic stem cell research) My question is...where does the president receive the authority to write executuve orders??? I thought he was limited by the powers of the legislative branch.

These, of course, have come from the evolution of American history, stemming in the tradition of Lincoln's Emmancipation Proclamation and Roosovelt's New Deal policies. The president has been granted through changes in the government far more "imperial" powers that the Constitution grants.