Major modules for which a quick summary can't do justice

Organic groups, aka OG (
Enable users (or site builders) to create and manage groups (or semi-independent site sections). Groups may be private, public, or include a mixture of the two. List of (many, many) related modules:
Panels (
Requires Chaos tool suite.
"Allows a site administrator to create customized layouts for multiple uses" from within Drupal, using a drag and drop interface.
Panels Everywhere (
Requires Chaos tool suite.
An "advanced method to [replace] Drupal's … blocks system and instead use the … Panels Layout system to control how your pages look."