The following applies only to version 3.x.
Earlier versions are no longer maintained.
Activity Stream builds a lifestream, a "River of You", by aggregating all your social activities in one place. Whether it's bookmarks on Del.icio.us, posts from Twitter or your blog, edits to wikis or enjoyed music and movies, anything you create can be gathered into one easy-to-read stream. Every item becomes a full Drupal node, allowing them to be searched, promoted, commented upon, and managed just like any other piece of content within Drupal.
Activity Stream 3.x also attempts to archive these activities by retaining a copy of the raw data received. This allows you to recreate your activity items, even if the remote site is no longer listing that item, if you didn't use all the data the first time around (GPS, user-agent strings, keywords, etc.), or if the service doesn't exist any more. If you uninstall the module, the imported content is kept, though all module configuration is deleted.