The basic account is free to use. Upgrades are available at a monthly cost.
Mendeley is an online reference management tool, which also allows you to share documents and collaborate with others.
To get the most from the product, you should download the desktop version of Mendeley, and also the Save to Mendeley button for your browser.
There is a range of help available from the Mendeley site.
There is a Mendeley blog which gives details of developments on the platform.