The antlered God of Gaul. Except for images and one pillar where He is mentioned by name, we know nothing of this God. He is most often depicted with antlers on his head, sitting crosslegged and accompanied by a ram-horned serpent. Often wearing or holding a torc and with a bag of some kind in His lap. In some depictions He is surrounded by other animals.

The antlers and animals may point to Cernunnos being a deity of nature, specifically of the hunt and the forest.
The horned serpent is often tied to the Greek Cerastes: a horned snake which likes to burrow deep into the earth. Thus linking Cernunnos to the Underworld and perhaps making Him also a deity with Chthonic roots.
The bag on his lap is believed to be a coin purse, linking Cernunnos to trade and to civilization. On the Gunestrup Cauldron He is also depicted with dogs, leading to speculation that Cernunnos might not be the God of wild nature, but instead of taming nature to provide commerce and trade.
Statues and carvings of three-headed Gods, like the Condat-sur-Trincou have also been linked to the God Cernunnos. Most had either carvings of trees and leaves, while some had removable antlers. Because of the three heads Cernunnos is thought to be linked to liminality; the in-between, as well as uniting opposites.

In Wicca Cernunnos is often named as their main male deity: the Horned God. This has caused some conflation with Him and some other Horned deities, like Pan. In Wicca, and because of this now outside Wicca as well, Cernunnos is seen as a God of sexuality, of revelry, of the Sun, who is the consort of the Triple Goddess and goes through a cycle of death and rebirth throughout the year.

To me, Cernunnos is the God of Nature, of the Wild and in-between places. He is the crackling bonfires that light up the darkest night. He is the Wild Hunt. He is the spark of sensuality and the rush of protective aggression. He is the stillness of the forest. He is the stag and the wolf, both hunter and hunted.

Photo by Kinsey on Unsplash

Further reading:
My Journey to Cernunnos
Sacred Space
Drawing the God and Goddess
Burnable Spellboxes