Preprints are preliminary reports of work that have not been certified by peer review. They may or may not be quality research. The researchers who post their work to these websites may be posting information that has been accepted to a journal, but not published yet (thus the name "preprints"), or rejected by a journal for a variety of reasons. Some researchers post to these sites in an effort to sidestep the business of academic journals, or simply to share their findings as soon as possible. Use careful discernment when considering whether to use this research.
A white paper is an authoritative report or guide that informs readers concisely about a complex issue and presents the issuing body's philosophy on the matter. It is meant to help readers understand an issue, solve a problem, or make a decision.