"Pagan" is an umbrella term that covers a wide variety of religions, everything from Asatru to Zoroastrianism to be exact.
Wicca is a modern religion which is based on the pre-Christian religions of northern Europe. Wicca in it's original form is an initiatory, Mystery religion, which is oathbound, practiced in a coven and is a priesthood. With the rise in popularity of Wicca through popular culture and the increase in publications on the subject, it is now fair to say that there is Wicca and a number of paths based on Wicca. Wicca itself remains the initiatory form and the latter comprises those paths inspired by the first form, which are not necessarily initiatory but are often solitary or eclectic and often involves a self-dedication ritual by the practitioner to their chosen deities.