Mendeley is a reference manager owned by the company Elsevier, a large scientific publishing company. Mendeley was originally open-source and predicated on the ability to share and reference full-text articles between research communities, both publicly and privately. Since being bought by Elsevier, Mendeley has been going through some changes to their platform. The Mendeley program for managing citation is called Mendeley Reference Manager. Previously, their program was called Mendeley Desktop and you can still download this older version until September 2022. Read more about Mendeley's move to their new program in their blog post Introducing Mendeley Reference Manager.
There are three parts of Mendeley to download: