Cernunnos is primarily a god of the Gaulish Celts. One has to realize that the Celts did not have one single mythology or pantheon, but that each tribe or tribal area had their own particular beliefs. Cernunnos does seem to have been worshiped over much of the Celtic areas of Continental Europe.
In Britain there is a similar deity called Herne. But there is some question whether Herne was a Celtic deity or one which came along later. Or perhaps there was such a deity and the name Herne was given to him because the name that the Celts called him by was long lost.
In Welsh mythology the deity that fills the role of Cernunnos (although he has no antlers) is Gwynn ap Nud. And in Irish mythology that role is taken by the Dagda.
Other cultures also had their versions of the Horned God such as Pan, Tammuz, Faunus ...and even in some statues, Jehovah.
In Celtic imagery, the horns are a symbol of royalty or kingship. This is further supported by the torc that he holds in his hand. The animals which are generally seen surrounding figures of Cernunnos indicate the God's ties to the animal world. Thus he is sometimes known as the Lord of the Animals or the Lord of the Hunt. There is an excellent discussion of the symbolism associated with Cernunnos at http://www.pauldburley.com/cernunnos.html
As for his relationship to Wicca, that is an indirect one. The paired deities in Wicca are a Moon Goddess and a Horned God. In Traditional Wicca the names of these deities is oathbound and only revealed to someone properly initiated into a Traditional Wiccan coven.
If one is not part of a Traditional Wiccan coven then you can determine by yourself through your personal experiences with the Divine which Gods you might be working with. If it is Cernunnos that calls to you, then he is certainly an appropriate form of the Horned God for you to work with.
Some other sites with interesting information on Cernunnos include:
The Green Man, who was mentioned as being a form of Cernunnos, is probably something quite different. He is certainly a symbol of seasonality and the turning of the Wheel. But he seems to have arisen in the Middle Ages during the Christian period and not so much during the Pagan period. That being said, he has been taken up by modern Pagans as symbolizing the regeneration of the earth each year.