EndNote has a Desktop version which must be installed on every computer where you want to use EndNote Desktop and an Online version which can be used from any computer connected to the Internet.
The two versions can be synchronised and shared between multiple users.
Synchronising allows you to have the same EndNote library on every computer where you use EndNote Desktop.
Sharing allows you to collaborate with 100 other users to create a combined EndNote library for your projects.
Anyone can sign up for an EndNote Online Basic account.
It is strongly recommended that you install EndNote Desktop first and then create an EndNote Online account from EndNote Desktop. This will create a fully licensed EndNote Online account which has more features and higher storage limits than the basic account.
For a comparison between EndNote Online Basic and EndNote Online Licensed see http://endnote.com/product-details/basic