Displaytag comes with 2 different tlds. Look at this table to understand which is the right one for you.

tld URI description
displaytag.tld Jsp 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.tld EL version of the tag library. It offers the same features as the
standard version, plus Expression Language Support. It will
require a couple of addictional libraries, see the dependencies
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 tld and still have
EL support (the EL tld will not work, since expressions will be
evaluated twice).

Jsp 1.1 / j2ee 1.2

Note that as of displaytag 1.1 the jsp 1.1 version of the taglibrary is no more supported.

If you need to use displaytag with container that only supporting j2ee 1.2 (Tomcat 3, Websphere 4, WebLogic 6...) you will have to stay with displaytag 1.0.