What is a PostDoc?
Postdoc: A postdoctoral scholar (“postdoc”) is an individual holding a doctoral degree who is engaged in a temporary period of mentored research and/or scholarly training for the purpose of acquiring the professional skills needed to pursue a career path of his or her choosing.
It is estimated that there are approximately 89,000 postdocs involved in research in the United States (National Science Foundation, Science and Engineering Indicators 2008). The number of postdocs in the U.S. has been steadily increasing due to the fact that the postdoc position has become the de facto next career step following the receipt of a doctoral degree in many disciplines. In these positions, postdocs typically perform research under the supervision and mentorship of a more senior researcher, often called the postdoctoral advisor. The key characteristic of a postdoc position is that it is a temporary career-building step on the path to a more permanent position.
Postdoctoral appointees can pursue basic, clinical or translational projects so long as their primary effort is devoted toward their own scholarship. Postdocs are essential to the scholarly mission of the mentor and host institution, and thus are expected to have the freedom to publish the results of their scholarship.
from – National Postdoctoral Association [http://www.nationalpostdoc.org/policy-22/what-is-a-postdoc]