This article is from the Wicca FAQ, by firstname.lastname@example.org (Rain of Teleport) with numerous contributions by others.
The coven is the basic, cellular "congregation" for some Wiccans, but is
often very formal, selective and closed, aiming for an ideal of "perfect
love and perfect trust" among members. Most Wiccans begin in less formal
ways such as attending festivals, public rituals, classes or more open
groups (often called "circles"). Many Wiccans probably begin and continue
practice as "solitaries," whether before, after or while a member of a
coven. Solitary practice is a valid "tradition" in the Craft, but some
good places to find other Wiccans are on the net, at public Pagan events or
through occult, political or "new age" bookstores.