Paganism can be at the top, but isn't always. Some people put other things above it, such as occultism, new age beliefs, etc. But there are many religious paths that do fall under the category of "paganism," and Wicca tends to be one of them.
"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.
Actually that is not a good analogy. Catholics are Christians and as Christians they share a set of core beliefs with all other Christians.
Within Paganism there are many different religions all of which often have very differing beliefs. In fact it's hard to come up with specific shared beliefs within Paganism. Perhaps the closest one could come is that most (but not all) tend to be polytheistic, most (but not all) have no central authority, and that most (but not all) honor some form of the Divine Feminine. Beyond that the comparisons get really murky.
Pagan:
A person holding religious beliefs other than those of the main world religions.
Of or relating to such people or beliefs: "a pagan god".
one who has little or no religion and who delights in sensual pleasures and material goods : an irreligious or hedonistic person
Wiccan:a religion influenced by pre-Christian beliefs and practices of western Europe that affirms the existence of supernatural power (as magic) and of both male and female deities who inhere in nature and that emphasizes ritual observance of seasonal and life cycles