An IFilter is a plugin that allows the Windows Indexing Service and the newer Windows Desktop Search to index different file formats so that they become searchable. Without an appropriate IFilter, contents of a file cannot be parsed and indexed by the search engine. IFilters are also used by SQL Server as a component of the SQL Server Full Text Search service.

IFilter is designed to help users to index a large number of documents and then quickly find text within these documents. The documents can be files, email attachments or database records.

Windows Indexing Service, 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. Hereafter, we use term "search engine" to refer to these products.

An IFilter acts as a plug-in for full-text 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 in windows registry for an appropriate IFilter and finds the IFilter. IFilter understands the document format. It filters out embedded formatting and extracts text from the document and return text back to the search engine.