Mendeley is a free, easy-to-use tool that helps you collect, organize, cite, and share your research sources (variously called "references," "citations," "documents," or "files").
It automatically updates periodically to incorporate new online sources and new bibliographic styles.
There are other reference managers, notably Zotero and Endnote, so why use Mendeley? If you are looking for a product with robust features related to managing and working with PDFs, Mendeley may be the reference manager for you.