TagLibs are an extension of JavaServerPages.
They introduce custom XML tags which are interpreted by Java classes at execution time.
The original specification can be found at http://java.sun.com/products/jsp/taglibraries.html.
http://java.sun.com/products/jsp/jstl/ since http://java.sun.com has changed.
TagLibs is also the name of one of the JakartaProject's subprojects (http://jakarta.apache.org/taglibs/).