The document discusses the purpose and differences between the multiple lib directories in Tomcat 5.5.x. The <Catalina_Home>/server/lib directory contains jars only visible to and used by the Tomcat container. The <Catalina_Home>/common/lib directory contains jars visible to and used by both the container and web applications. The <Catalina_Home>/shared/lib directory contains jars only visible to and used by web applications. Each directory is loaded by a different class loader that determines which components the jars are visible to.