This update site provides the features of the Eclipse Web Tools Platform (WTP).
If you are just getting started with Eclipse or Eclipse Web Tools, you may want to first install from the Ganymede update site, as it would have everything you need to get started.
This Web Tools Platform update site does assume you have all the pre-reqs installed.
Beyond the basic Web Tools Platform, this site offers the SDK version of Web Tools and also the XSL Incubating Component.
To use this update site in your existing installation of Eclipse, add the following URL as a remote update site in your update manager, if it does not already have it.
http://download.eclipse.org/webtools/updates/
See Eclipse Help for general information about installing and updating Eclipse.