EndNote Basic
EndNote Basic is a free web only version of EndNote available at myendnoteweb.com. It is not as robust as the desktop version, and has many less citation styles available for use.
EndNote Web/EndNote21
EndNote Web is only available to those who have purchased EndNote 21. The interface is much more similar to the desktop version. It is a completely different access point than EndNote basic at https://web.endnote.com/.
It has all the same citation style and storage features as EndNote 21 desktop version.
Syncing
You can sync your EndNote Basic or and EndNote Web databases using the sync tools within EndNote. This will merge your data from EndNote and EndNote Web.
Syncing in older versions of EndNote can ocassionally be buggy. If you are having trouble syncing, you may want to export your EndNote database and then upload it to EndNote Web.