The Mendeley.com library of citations is crowd-sourced. What this means is that all the citations added to your personal Mendeley library are also added to the database of citations that Mendeley shares with everyone. This is what drives the discovery aspect of Mendeley--you can find new research based on what your colleagues and other scholars have added to their libraries. An important note: all citations are anonymized, so your citations will not be tied to your name or account in Mendeley.com.
There may be times when you don't want a citation in your personal Mendeley library to be added to the crowd-sourced library. For example, this could be unpublished papers, personal manuscripts, or class notes. To keep these citations private, be sure to check the "Unpublished work" button at the bottom of the document details form. For more information, follow this link.