Lightly Obscured: Squares of dim light, foliage, fog, smoke, heavy falling snow, or rain are lightly obscured.
Concealment: A target in a lightly obscured space has concealment.
Heavily Obscured: Squares of heavy foliage, heavy fog, or heavy smoke are heavily obscured.
Concealment: A target in a heavily obscured space but adjacent to you has concealment.
Total Concealment: A target in a heavily obscured space and not adjacent to you has total concealment.
Published in Dungeon Master's Guide, page(s) 61.