Lucene is an open-source search library that powers search capabilities for many consumer and enterprise applications. It allows applications to index, search, and retrieve documents from various sources and formats. The document discusses how Lucene is used in search scenarios like document retrieval and routing. It provides examples of consumer-facing sites and enterprise applications that use Lucene, including Google, Facebook, LinkedIn, Apple, Cisco, Goldman Sachs, and more. The document also outlines Lucene's capabilities to aggregate, extract, analyze, and index data and knowledge for enterprise search platforms.