Displaytag comes with 3 different tlds at the moment. Look at this table to understand which is the right one for you.

displaytag-11.tldhttp://displaytag.sf.netJsp 1.1 version of the tld, you will need to use this one if you plan to install your application on container only supporting j2ee 1.2 (Tomcat 3, Websphere 4, WebLogic 6...).
displaytag-12.tldhttp://displaytag.sf.netJsp 1.2 version of the tld: requires j2ee 1.3 (Tomcat 4, WebSphere 5, WebLogic 7...). Use this version if you are not looking for j2ee 1.2 compatibility and don't need EL support.
displaytag-el-12.tld version of the tag library. It offers the same features as the standard 1.2 version, plus Expression Language Support. It will require a couple of addictional libraries, see the dependencies page. Don't use this one if you are looking for EL support on jsp 2.0 containers (Tomcat 5). In Jsp 2.0 compatible servers expressions are evaluated directly by the container, so you can use the standard 1.2 tld and still have EL support (the EL tld will not work, since expressions wil be evaluated twice).