When you create a zotero.org account, you get access to a web version of your library. This can be used to access your library on a different computer or mobile.

The web version of the library gives you access to most Zotero features.
How to transfer references between Zotero and Endnote:
Zotero to EndNote
EndNote to Zotero
If you have a text file (for example, a Word Document) of references that you would like to import into Zotero, you can use AnyStyle to parse them into a format Zotero can import.
You can open PDFs you have saved in Zotero standalone by double clicking. The PDF will open in a new tab.

In the PDF reader, you can highlight text, highlight an area, or add a note using the controls at the top:

You can also annotate your highlights by clicking on them:

An active community of Zotero users has developed a variety of plugins to provide enhancements, new features, and interfaces with other programs.
Installing Zotero Plugins
The first step is to read the installation instructions on the plugin page. Locate the link to access the current version of the plugin. Download the .xpi file, then go to Zotero standalone. In the Tools menu, choose Add-ons.
In the Add-ons Manager, select the gear icon and choose Install Add-on From File. Then select the .xpi file. You can also just drag and drop the .xpi file into the Add-ons Manager.

Go through any install prompts, and then your extension will be available.