This tileset replaces most of the landscape in the orc swamp tileset; you need the expansion to play it. It replaces the mud and land with a different type, replaces the water with weedy water, and changes the trees into giant eggplants. It also replaces the oil platform and goldmines to the summer set because it otherwise looks stupid.
To install it, simply run the exe file in your War2 directory and answer all questions with "yes".
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.