That is a common and easily asked question that doesn't have an easy answer. No siree, you can't answer that question in one or two sentences. It takes some thought and a thorough explication to explain to someone what Wicca really and truly is, and even then, if you don't practice it, you may not fully understand.
That said, let's jump right in!
Wicca is one of the many Pagan religions of the world, the word Pagan traditionally meaning one who is not Christian, Muslim, or Jewish. Wicca is a faith that has a many paths as it does followers, so the definition of what Wicca really is and what it really means will vary slightly depending on who you’re asking. These variations in “definition” are not because of a big, international cover-up of Wicca or because of some need to hide the “mysteries” of the Wiccan faith. These discrepancies are because of the fact that Wicca itself is still evolving and is growing at a tremendous rate.
Now, obviously, Wicca has many facets, many aspects. Within the faith there are many different Traditions (similar to the denominations of Christians), each following a similar though ultimately slightly different path. These Traditions are all valid, for they all have the same goal of creating and maintaining a personal, intimate relationship with the Lord and Lady, and then worshipping and celebrating in an appropriate manner.
The Wiccan path is a modern, dynamic religion. While it has numerous ties to the ancient practices in pre-Christian Europe, Wicca is not just the rehashing of those same religious systems and beliefs. Contemporary Wicca has taken the knowledge of its predecessors and given rebirth to them in a different way that speaks in a stronger manner to the people of today.
Wicca is a deeply personal spirituality that is crafted and shaped by each of its individual practitioners. Unlike various other faiths where the majority of followers can attend church once a week, pay a tithe, and then rest assured that their soul is saved for another week, Wiccans are required to be active and involved in their faith by learning, participating, and changing. The are also encouraged to ask questions, search for answers, have doubts, satisfy those doubts, and draw their own conclusions and beliefs, for we know that in the end our doubts and questions will ultimately make us stronger in our faith. Wicca does not spoon-fed regulation beliefs to its followers.
This faith focuses on Divinity (God and Goddess), also known as Spirit. The Lord and Lady are seen as two separate (masculine and feminine) while ultimately conjoined beings. They are separate yet the same, two while one. Wicca recognizes that we as humans are a part of Nature, not the masters of it. Therefore we seek to find our role in Nature by celebrating the changing of the seasons, the harvests, the phases of the moon, and the subtle energies and elements of the Earth.
It is an acceptance of the divinity in Nature, which was created by and is a part of the God and Goddess, and we practice various rites and rituals that recognize and communicate with that glorious divinity in all its manifestations. It is awareness of the Old Ones--the Lord and Lady-- as well as the elements and spirits who have always been here. It is a strong faith in the bond between us and the Divine.
Wicca is a loving path that focuses on creating a stronger, more intimate bond with the Divine and living a life acceptable to that path. It is a faith that encourages personal empowerment, merciful behavior, love, learning, and environmental responsibility. The rituals and faith of Wicca are irrevocably tied to the changing of the seasons and the phases of the moon, and Wiccans recognize that our intelligence gives us a responsibility to protect and care for the Earth which nourishes us.
While Wicca is religion that encompasses Witchcraft (the practice of using the natural energies of the Earth around us and within us to create change) as a means of practice and worship, always keep in mind that not every Wiccan is a Witch and that not every Witch is a Wiccan.
Wicca IS a federally recognized religion in the United States and is therefore protected by the Constitution. Wiccans have the same rights to belief, worship, and openness (like, being public, wearing religious jewelry, etc) as do Jews, Christians, Muslims, and everyone else of faith. Goddess bless America.
So, in recap, Wicca is a diverse faith that worships God and Goddess, is loving, compassionate, and empowering. Masculine is not favored over feminine, and vice versa. Balance and equality are major focal points in Wicca.
Now, one more real quick point. Wicca is NOT satanic. Wiccan do not recognize an incarnation of absolute evil such as satan. God and Goddess did not place evil into this world; They merely placed the potential for both good and evil. Humans brought evil into the world easily enough without the help of some evil demigod that God created to play cosmic war games with. Wicca does not sacrifice humans, animals, etc. Wicca is not a cult. We’re not here to brainwash anybody or take over the world.
We’re here to walk this path, just like everyone else.