Metadata or descriptive information (like title, author, category, update date...) about a document can only enrich the search experience if used correctly. Some content providers have categorizers on staff who associate metadata to documents, but this can be very costly. Some search engines can create some metadata for documents. Other specialized software packages (Entity extraction tools) can add additional metadata.

Structured vs. Unstructured - Approximately 80% of searchable documents have no metadata associated with them and are "unstructured".  The remaining 20% have some metadata (or structure) and advanced techniques can be used on these structured documents to make finding information in them easier.

Faceted (Parametric Search) - Faceted or Parametric Search is a technique, supported by many search engines where some combination of metadata and taxonomies combine features of Browse and, Navigation with Search. Many Search engines enable search, browse and drill in of content in an integrated manner facilitating discovery of information.

Indexing/Collection Build - This occurs when content is put directly into a "collection" or repository for direct searching. Content may be from Content Management Systems, file systems, databases, subscription feeds, RSS, the internet and from a variety of other sources.  This type of search is the most optimized for performance and scalability.

Federated (meta) Search - A federated search capability (or searching of multiple sites at one time where native search capabilities are used) is most often built when it is unfeasible to index content from sources because of volumes, accessibility, security, update frequency, license (or use) restrictions or other reasons.

Classification Tools - Some search engines support document classification. User Created Rules (Taxonomies) as well as Dynamic (or on the fly classification) can be used to categorize documents. 

Entity Extraction - Created by search or third party vendors, these tools add additional metadata (or attributes) to documents.  Some vendors have created specialized modules specifically for different vertical sectors (e.g. financial, pharmaceutical, banking, insurance...) which can add even more value when integrated with search

Enhanced Functionality - The most powerful search applications can add value for users with extra features including: