Zotero is a tool that helps you build your own library of items that you will use in papers, presentations, or posters and includes articles, books, government documents, images, web pages, and much more. Best of all, it connects with MS Word or Google Docs to easily add citations in the body of your paper and create a bibliography at the end in the citation style of your choice (or as prescribed by your professor).
Additionally, Zotero allows you to attach PDFs, add notes, attach any type of file, and organize them using folders (or what they call collections) or using tags (which you can colour-code). You can add items directly from a database (including Google Scholar, by dragging in a PDF to Zotero from your computer, or by manually entering the information.
To use Zotero properly, you need to install the Zotero Connector for Chrome, Firefox, or Safari, in addition to the Zotero desktop app.