definitions from Wikipedia
Celtic Wicca is a current of Wiccan neopaganism, loosely syncretized with elements of Celtic mythology, mostly, as noted by authors including Hutton, Kelly, Greer and Cooper, by way of the Romanticist Celtic Revival. Raeburn (2001) is aware of the ahistoricity of "Celtic Wicca", establishing "a firm distinction between historical Celtic inspiration and modern Wiccan practice". Followers practice meditation, divination, nature mysticism and "magickal herbalism". Emphasis is placed on the Celtic pantheon, history, traditions, food, and music. Celtic Wiccans occasionally call themselves "druids", putting themselves close to Neo-Druidism, which is likewise an outgrowth of the 19th century Celtic Revival. Wicca, as established by Gerald Gardner in the 1940s, contained a few Celtic elements, along with elements from many other cultures (Greer and Cooper, Hutton, Kelly); Celtic Wicca can be seen as emphasizing such Celtic elements as there are to be found in Gardnerian Wicca while de-emphasizing the non-Celtic elements.
Neo-druidism or neo-druidry is an attempt to construct a modern spirituality based on the ancient religion of the Celts, as presided over by the priestly caste of druids. A fundamental difference between ancient druidism and modern, or neo-druidism, is that present-day druids do not hold the prominent place in society that was enjoyed by druids in pre-Roman times. In general, Neo-druids promote the peace, preservation and harmony of nature. The original ceremonies of the neo-druids involved gathering in a wooded place periodically (usually weekly, but some groups used astrology to calculate meeting times), for the ritual consumption of "spirits" (Scotch or Irish whiskey blended with water) called "the water of life" (uisce beatha, or whiskey), the singing of religious songs, the performance of ceremonial chanting, and, occasionally, a sermon.
The written RDNA liturgy calls for a "sacrifice of life", reflecting the core of the Reform, namely plant rather than animal sacrifices, and (for the ordination of a priest) an outdoor vigil.
Specifically in the Mother Grove, the use of Scotch rather than Irish whiskey has been an ironic tradition dating from the first ceremony, at which a partial bottle of Scotch whisky had been at hand, left unfinished at the end of a party the previous night. The major holy days are the quarter days (solstices and equinoxes) and the solar festivals (approximately half way in between the quarter days, these are: Beltane, Lughnasadh, Samhain and Imbolc). These are celebrated with (usually outdoor) parties with a religious theme, much singing of religious songs, dancing in circles, etc. Various individuals will also have their own private ceremonies. Often, small groups will break off, and perform their own separated ceremonies before rejoining the general group - these groups are often split along initiatory lines as those of higher degree work their own ceremonies. Individual choice is a major theme. So is ecology, though more in the sense of being sensitive to it and living lightly on the land than in the sense of a study of the interrelationship of lives at various scales.
The major gods are, in RDNA liturgy, the Earth-Mother (addressed as "our Mother"), seen as the personification of all material reality, Béal, the personification of nonmaterial essence, and Dalon Ap Landu, the Lord of Groves. The first two are sometimes referred to as the Earth and the Sun (named in Gaelic). Some individuals prefer to devote most of their praise, however, to other gods, like Health or Music (usually also named in Gaelic). And "A Druid Fellowship" has various scholastic posts and honors, though usually in the arts as devoted to religious praise rather than as formal studies. ADF's liturgy is considerably more complex than that of the RDNA, though its roots in the older group are obvious, based on Bonewits's theories of a common pattern to Indo-European worship.
Neo-druidism is considered a neo-pagan religion. It is important, however, to realize that the founders of RDNA intended it to complement or supplement "organized" religion, not to supplant it; most of the founders were practicing Christians. They were very surprised when RDNA continued after the college repealed the religious attendance requirement. As someone put it, "Apparently our disorganized religion appealed to those who couldn't stomach organized religion!" Present-day adherents range from those who are exclusively Druids to those for whom it is, indeed, a complement to another faith.
Druidry: In Celtic polytheism the word druid denotes the priestly class in ancient Celtic societies, which existed through much of Western Europe north of the Alps and in the British Isles until they were supplanted by Roman government and, later, Christianity. Druidic practices were part of the culture of all the tribal peoples called "Keltoi" and "Galatai" by Greeks and "Celtae" and "Galli" by Romans, which evolved into modern English "Celtic" and "Gaulish". They combined the duties of priest, arbitrator, healer, scholar, and magistrate. The Druids were polytheists (worshiped morthan one god/dess), but also deified elements of nature, such as the sun, the moon, and the stars, looking to them for "signs and seasons". They also venerated other natural elements, such as the oak, certain groves, tops of hills, streams, lakes and even plants, especially mistletoe and holly. Fire was regarded as a symbol of several divinities and was associated with the sun and cleansing. Their calendar year was governed by the lunar, solar, and vegetative cycles. Archaeological evidence suggests that ceremonies were conducted to celebrate the two solstices and two equinoxes every year. These festivals would have been governed by the position and motions of the Sun alone. In addition to these, four holidays were celebrated according to the lunar and vegetative cycles. These include Imbolc (Imbolg) to denote the first signs of spring, Beltane (Beltain) to recognize the fullness of life after spring, Lughnasadh to celebrate the power of the Solar deity Lugh, and Samhain to recognize the lowering of the barrier between the world of the living and that of the dead. The timing for these latter four festivals would have been determined by the presence of a full moon and the signs of life implied by the above. Imbolg would thus be celebrated at a full moon roughly halfway between the winter solstice and the vernal equinox, Beltane between the vernal equinox and the summer solstice, Lughnasadh between the summer solstice and the autumnal equinox, and Samhain between the autumnal equinox and the winter solstice. This is contrary to popular "New Age" beliefs about Druidism that celebrate a given holiday according to the Julian calendar, which of course did not exist at the time of the formulation of these holidays. In modern times, Imbolg has been transformed into Groundhog Day, elements of Beltane have been absorbed into Easter, and Samhain has become Halloween (or All Hallows' Eve or All Saint's Day).
Modern attempts at reconstructing or reinventing Druidism are called Neo-druidism.