Citation managers help organize the citations you collect, and the social network aspect of Mendeley also adds the citations of its users into a big searchable database. Mendeley allows users to create groups and share the articles they use. PDFs can be annotated and notes kept in Mendeley Reference Manager.
Mendeley was created by three Ph.D students in 2007, and the company was founded in 2008. It was later sold to Elsevier in 2013*.
The basic storage plan (2GB) is free and includes the ability to create and support up to 5 shared group libraries of up to 25 users (100MB shared storage across all groups).
More storage is available through "Premium Packages" for a fee:
Styles can be downloaded within Mendeley Desktop by going to Citation Styles under the View tab, and then clicking on More Styles.
The screen shown here will let you search for the style you need and install it that way, or download via the source URL from one of the libraries below.
It will then appear in the citation style options when you export your items as a bibliography.