The Ruins Mod adds 9 different ruins to the game. They will generate at random in your worlds and usually there's some type of treasure, like a chest, which can be found inside the ruin. It makes exploring much more fun as there's a greater chance of finding something else than just trees, hills and forests.
Where do I find the ruins?
There are several different types. Some of them include a shipwreck, a miniature sand temple and some old stone structures. Shipwrecks will spawn in water and the other ones on land, usually in the biome it is related to.