Elgg is an open source social networking platform that allows users to build customized social networks. It provides features for private and public social networks, and is highly extensible through a plugin architecture. The document discusses how Elgg allows adding new features without modifying core code, through replacing or extending views, actions, and events. Data is stored as entities, metadata, annotations and relationships without needing SQL knowledge. Examples show how easily a blog post can be created and retrieved through the Elgg API.