Sanguina Cruenta
Wicca is an orthopraxic, oathbound, initiatory, experiential, fertility-focused, ditheistic mystery religion comprised of clergy.

What does this mean?

Orthopraxic
means that it's defined by practice, rather than belief. Christianity is an example of an orthodoxy. If you beleive the right things, you are a Christian. Wicca is an orthopraxy. What you believe is not as relevant as what you practise. If you practice the right things, you are a Wiccan.

Oathbound means that the bulk of what defines the religion - which, as we have already seen, is the ritual - is bound by oath and secret. Breaking these oaths and the secrecy involved will bring the Curse of the Goddess upon you. There are practical reasons for this secrecy, which we will get to later, although personally I reckon much of it is because Gardner was so into his secret societies and such and it was something he enjoyed.

Initiatory means that it is a religion practised in a group, that one needs to be formally brought into via initiation. This initiation brings one into the religion, and declares one as a witch and a priest or priestess. It introduces one formally to the gods. In this initiation, you experience the gods and a rebirth. You are told the names of your deities, take oaths to keep these secret and to defend your religion and so forth. The fact that Wicca is initiatory means that, however wonderful one's faith, if you haven't been initiated into a coven that traces its initiatory lineage (High Priestess --& High Priest --& HPS --& Gardner) all the way back to Gardner, then you are not a Wiccan.

Experiential
means that you are not told about Wicca. The beliefs you hold, you form during the rituals in which you participate. No one will sit you down and tell you what Wiccans believe. Instead, by practising the rituals, you experience the religion, the gods, and the mysteries (we'll get to that later) and thus form the appropriate beliefs. For this reason, if you are not practising the rituals of Wicca, you cannot be a Wiccan, because you haven't experienced the religion. This is why Wicca is orthopraxic. Additionally, this is why Wiccan ritual is oathbound. To properly experience the religion, it must be new to you. You cannot understand it properly without training and context, so reading or being told a ritual will mean very little to you, and if you choose to become a Wiccan later on, knowing the bare bones of a ritual will detract from that experience for you.

Fertility-focused means that Wicca is involved in the cycles of life, sex, death, rebirth. The fertility of the land, and so forth, but also importantly the fertility of humanity. We understand fertility subjectively through our own bodies. So sex is involved in Wicca. Yes, it is. Rituals are performed naked 98% of the time, and initiations are always naked no matter what. There is sexual contact in many rituals, and full sex in the Great Rite and third degree initiation. As an aside, we cannot say that Wicca is a nature religion. Yes, fertility is part of nature... but in the same way that a square is a shape. It's part of nature, not all of nature. Wicca is concerned specifically with fertility.

Ditheistic means that Wiccans worship two deities, the Lord and Lady of the Isles. Now, many Wiccans also worship other deities as well, as the deities of Wicca are not jealous. But, these other deities are worshipped outside the context of Wicca. It is a slight to Wicca to bring other deities into Wiccan ritual, and it is a slight to those deities to try to shove them into roles to which they are not suited. How individuals understand the Lord and Lady of the Isles will vary, as Wicca, as mentioned, is concerned with experiential ritual, rather than belief.

Mystery religions are secret. They are involved with a series of "mysteries" concerned with the natural world and the gods. These "mysteries" cannot be told or explained in words; instead, they must be understood through experiencing them. Someone can tell you a mystery in words, and you can nod and think you get it. Then, years later, you will experience something that causes an epiphany and suddenly you actually get it, on a deep, soul-deep level, and you understand it with all of yourself. That is what mystery religions try to do. They have specific mysteries that they attempt to teach, through their initiations and experiential rituals, by trying to replicate the situations in which you "get it".

Comprised of clergy means that each and every Wiccan is a priest or a priestess. There are no lay people in Wicca like there are in most religions. Each Wiccan not only interacts with but directly serves their deities. Priesthood is a heavy thing, and requires mental, emotional and spiritual maturity and strength. For this reason, and for others, Wicca is not for everyone.