What is a Preprint Server?
A growing number of disciplines are building online repositories where scholars can deposit their preprints, or manuscripts of articles submitted or accepted for publication. Preprint servers permit rapid dissemination of new research output that is openly accessible to all readers. These preprint servers differ from institutional repositories like Northeastern's Digital Repository Service (DRS) in that they accept deposits from scholars at any institution. They're organized around a research discipline rather than a specific research institution.
Nonprofit preprint servers also differ from commercial services like ResearchGate and Academia.edu. The former are organized by nonprofit entities for the benefit of the research communities they server. The latter are social networking sites that have commercial purposes for gathering user data. See "A Social Networking Site Is Not an Open Access Repository" from the University of California for more insight on this distinction.