The jungle tile set will replace your forest tile set. There will be exotic plants and trees instead of the normal ones. The dark ground will be replaced by grass which may be a bit confusing. There are no changes in the gameplay (no new AIs).
To install it, simply run the exe file in your War2 directory and answer all questions with "yes". If you have problems, use a fresh maindat.war file (copy it from the CD to your War2 directory).
This tileset has been made by Hellmage.
To get your old tileset back, either copy maindat.war from the CD into the war2\data directory or make a backup of that file first.