The old gods of the children of the forest are nameless deities of stone and earth and tree, which were named so by the followers of the Seven ("New Gods") that replaced them in all but North of Westeros, where it's still practiced strongly by Northmen, Crannogmen and Free Folk from Beyond the Wall.
The old gods appear to be a sort of animistic religion. The greenseers of the children, shamans of a kind, were said to be able to talk with all beasts and birds, and to see through the eyes of their carved weirwoods and posses strong magic.
There are no priests, no holy texts, no songs of worship, and practically no rites that go with the worship of the old gods. It is a folk-religion, passed from generation to generation. The closest thing to a ritual we have seen is prayer before the heart tree in a godswood, holy groves contained within castles throughout the Seven Kingdoms, and often the only places where living weirwoods still remain until one goes north of the Wall. It is said that the sigh of the wind and the rustle of leaves are the old gods speaking back to worshippers.  The children of the forest believe that the weirwood trees were the gods and when they died they become part of the godhood. 
Weirwood trees with faces carved into them, called heart trees, are considered sacred. Prayer, oaths, and marriages are often performed in the presence of a heart tree. The faces were carved into the weirwoods by the children of the forest, but their meaning or purpose is not completely understood by humans. Once all noble houses had a godswood with a heart tree in its centre, but many families that no longer follow the old gods have converted their godswoods into secular gardens. It's said that the old gods only have power where the heart tree faces can see, and since the destruction of most of the heart trees in the south they have no power there.
The old gods were originally worshipped by the children of the forest in all of Westeros since before first recorded history, thousands of years before the arrival of the First Men from the east. The First Men warred with the children, and cut down the weirwoods where they found them. In time, the First Men made peace with children of the forest and adopted their gods.
Worship of the old gods remained strong across Westeros until Andal Invasion, who brought their Faith of the Seven with them from the east. The Andals gradually conquered the south of Westeros, cutting down the weirwoods supplanting the worship of the old gods with their own, with the exception of House Blackwood in the Riverlands. The old gods are still worshipped in the North, and by the wildlings beyond the Wall.
Worshippers of the Seven have been known to convert to the old gods. A convert to the old gods is Samwell Tarly, a southron of the Night's Watch who was originally of the Faith of the Seven but chose to take his vows with Jon Snow in front of the nine weirwoods in the Haunted Forest. When Sam looks at the faces on each of the trees, and says that he can feel the gods.
References and Notes
- Many Gods & Dark Faiths article by Ran and Linda
- A Dance with Dragons, Chapter 36, Daenerys VI.
- A Clash of Kings, Chapter 34, Jon IV.
- A Clash of Kings, Chapter 52, Sansa IV.
- A Storm of Swords, Chapter 24, Bran II.
- A Storm of Swords, Chapter 57, Daenerys V.
- A Game of Thrones, Chapter 53, Bran VI.
- A Storm of Swords, Chapter 46, Samwell III, p 529.
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