Tom Dalton Messages: 12 Registered: September 2009
I believe its an archive file (e.g. a .zip file) that contains an update site directory structure: generally a top-level site.xml file and then directories like features, plugins etc.
It's a way of getting a set of features into eclipse without needing an internet connection (e.g. the update site archive can be downloaded and burned to a CD, then installed via the CD somewhere else).