Example:
<%@ taglib uri="http://struts.apache.org/tags-html-el" prefix="html" %>
If you're using the non-el version of the tags, the URI will be different-- look in the tld itself for the <uri>http://struts.apache.org/tags-html</uri>, and use that in your <%@ taglib> tag.
The above method works because the JavaServer Pages specification requires the container to search inside the .jar files and the filesystem for those tld files.
If you elect to configure your TLDs in web.xml, then to take full advantage you can tell Tomcat *not* to search for tlds with
processTlds="false"
See: http://tomcat.apache.org/tomcat-5.5-doc/config/context.html#Standard%20Implementation