At least one branch of neo-druidism began at Clarendon College, in response to a requirement that all students participate in a religion. It was originally not intended very seriously, and nearly died out after the requirement was removed. Robert Larson was ordained during that period, and later carried the history to Berkeley California, where he encountered Isaac Bonewits. Together they founded a small congregation with affinities to various Wicca groups and to various practitioners of ceremonial magic (or Magick if they were Crowleans). Since then it has had several periods of greater or lesser activity. Currently the most visible group is "A Druid Fellowship" (the actual name is Gaelic). They are centered on the East Coast of the United States, in New York and New Jersey, but with branches scattered throughout the rest of the US as well.
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" (Irish Whiskey blended with water) called "the water of life" (uiskibaugh, or whiskey), the singing of religious songs, the performance of ceremonial chanting, and, occasionally, a sermon.
The major holy days are the quarter days (solstices and equinoxes) and the cross-quarter days (half way in between the quarter days). 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. Also, small groups will break off, and perform their own separated ceremonies before rejoining the general group. 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.
Generally the major gods are the Earth and the Sun (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.
Neo-druidism is considered a Neo-pagan religion. See Neopaganism page for other Neo-pagan faiths.