An IFilter is a plugin that allows Microsoft's search engines to index various file formats (as documents, email attachments, database records, audio metadata etc.) so that they become searchable. Without an appropriate IFilter, contents of a file cannot be parsed and indexed by the search engine.
They can be obtained as standalone packages or bundled with certain software such as Adobe Reader,[Note 1] LibreOffice, Microsoft Office[Note 2] and OpenOffice.
It also refers to the software interface needed to implement such plugins.
An IFilter acts as a plug-in for extracting full-text and metadata for search engines. A search engine usually works in two steps:
During Step 1, the search engine itself doesn't understand format of a document. Therefore, it looks on Windows registry for an appropriate IFilter to extract the data from the document format, filtering out embedded formatting and any other non-textual data.
Windows Indexing Service and the newer Windows Search, Windows Desktop Search, MSN Desktop Search, Internet Information Server, SharePoint Portal Server, Windows SharePoint Services (WSS), Site Server, Exchange Server, SQL Server and all other products based on Microsoft Search technology support indexing technology. Also, IFilters are used by SQL Server as a component of the SQL Server Full Text Search service.