Wicca is a specific pagan religion. Wiccans belive in two deities, the Horned God and the Goddess. Lark has an amazin forum post about what Wicca is and isn't that I would highly recommend any who is interested in Wicca to read.
Pagan decribes any religion that has multiple deities. All wiccans are pagan. Not all pagans are wiccan.