Here is the beginning of a Wikipedia article on what PIE is:
The Proto-Indo-European language (PIE) is the unattested, reconstructed common ancestor of the Indo-European languages, spoken by the Proto-Indo-Europeans. The existence of such a language has been accepted by linguists for over a century, and there have been many attempts at reconstruction. Nevertheless, many disagreements and uncertainties remain. link I leave it to the experts to point out any flaws.
And here is part of an article I read with great interest:
Kathleen Hubbard's answer to the question "How do we know what we know about Proto-Indo-European and other languages that died out before they were written down? [Kathleen is assistant professor of linguistics at the University of California, San Diego. She describes herself as a "recovering Indo-Europeanist."] link
