Zotero (pronounced "zoh-TAIR-oh") is a program that interacts with your web browser to manage and cite research. You can attach PDFs, notes and images to your citations, organize them into folders, and create bibliographies.
It was first designed for Firefox, but now works with Safari, Chrome, and Edge, and both a stand-alone desktop application and a web interface help you manage and save citations on any computer.
See also this quick start guide published by the Zotero developers. Also available as a PDF.
Zotero has produced several great how-to videos on their site that demonstrate step by step how to use Zotero's features. Below are a series of videos from McGill Library about getting started with and using Zotero.
Jason Puckett's book, Zotero: a Guide for Librarians, Researchers, and Educators, provides a thorough understanding of how Zotero can be used in your research. His Zotero LibGuide gives a comprehensive view of the features of the software and how to use it in your library research. Some highlights include: