The National Bureau of Economics Research defines working papers as "intended to make research available to other economists in preliminary form to encourage discussion and suggestions for revision before publication." Working papers are extremely important in Economics research.
It's a good idea to check authors' websites, or one of the articles databases such as EconLit, to determine if articles that began as working papers have now been published.
Nonprofit economic research organization develops new statistical measurements, estimates quantitative models of economic behavior, assesses the effects of public policies on the U.S. economy & projects the effects of alternative policy proposals. The searchable index of NBER contains the full text of working papers from November 1994-present.