If you want to use EndNote Online with Cite While You Write, but don't want to install the EndNote program, you can download the EndNote Online Plug-in by logging into EndNote Online , hovering over "Options" and then clicking "Download Installers".
Note: If you use EndNote Online without the desktop your storage space for file attachments will be limited 2 GB.
If you use EndNote Online with Firefox, you can download a Firefox Extension that will let you save online references directly into your library by logging into EndNote Online, hovering over "Options" and then clicking "Download Installers"
EndNote is an application that runs on your computer. EndNote Online is web-based. They do many of the same things (save citations, create bibliographies automatically in Microsoft Word, etc) but have some differences. Check out the comparison of features on page 8 of the EndNote Online Quick Reference Guide for information on the differences.