The Old Gods are the gods of the trees, rocks and earth, originally worshiped by the Children of the Forest and later the First Men. They were named Old Gods once the Andals and their Seven (also known as New Gods) arrived and took over the South.
Following the Old Gods is represented in game by the Old Ways aligment.
To mark their places of worship the Children carved faces in trees, especially Weirwoods, since they have red saps which resemble blood. Houses which still worship the Old Gods often have a Godswood with one or multiple Weirwoods.