Mendeley is a free application (available for Windows, Mac, Linux, iOS, Android) that helps researchers manage and annotate their research paper collections. Researchers can also set up collaborative projects, work and discuss in groups, and share data on Mendeley’s web platform.
Mendeley is an Elsevier product, and accounts are shared across many of their products. So if you already have an Elsevier account, you can use the same login information. Similarly once you create a Mendeley account, the same login details will work for their other products, such as Knovel, Engineering Village or Science Direct.
The Mendeley Support Center has an FAQ that is a helpful resource.
The video What is Mendeley?, provides a nice overview of not just itself but the capabilities of citation managers in general.
The video Organizing your Library, covers topics including navigating the desktop application, modifying the resources' information/metadata, folders, categories for favorites or tags, the PDF reader and utilizing the highlighting and note taking features.