From DwarfFortressWiki

A ruin is a former settlement that has been destroyed in a war, rampage, or other reasons. As of 40d, there are no longer undead in ruins nor humanoid undead anywhere in the game.

[edit] Dwarven

A dwarven ruin is an empty mountainhome. Appears same as populated one, minus dwarves.^Verify

[edit] Human

Human ruins will be as generated, but buildings' floors and walls will be replaced with furrowed sand/dirt.^Verify

[edit] Elven

Elven ruins do not appear on the map, because elves do not make constructions.

Overworld
World generation - Regions - Biomes - Civilizations - Map legend - Sites - Armies
Aquifer - Brook - Cave - Chasm - Forest - Island - Lake - Mountain - Ocean - Pit - River - Tunnel - Volcano - Waterfall