A postprint is a digital draft of a research journal article after it has been peer reviewed. A draft before peer review is called a preprint. Postprints may sometimes be the same as the published version, depending on the publisher.
Since the advent of the Open Archives Initiative, preprints and postprints have been deposited in Institutional Repositories, which are interoperable because they are compliant with the Open Archives Initiative Protocol for Metadata Harvesting.
Preprints + postprints = "eprints". Eprints are at the heart of the Open Access initiative to make research freely accessible online. Eprints were first deposited or self-archived in arbitrary websites and then harvested by virtual archives such as citeseer (and, lately, Google Scholar), or they were deposited in central disciplinary archives such as Arxiv or PubMed Central.