XSL-FO, or Extensible Stylesheet Language Formatting Objects, is a markup language used for formatting XML data. It allows developers to create documents that can be transformed into various output formats, such as PDF or print. By defining how elements should be displayed, XSL-FO provides a way to control layout, fonts, colors, and other visual aspects of the document. XSL-FO is part of the broader XSL (Extensible Stylesheet Language) family, which includes XSLT for transforming XML data. It is commonly used in applications that require high-quality document generation, such as publishing and reporting systems.