In Wicca a coven is a tightly knit group who regularly worship together and act as a sort of magical family.
Covens are ideally composed of an equal number of men and women (plus one extra, traditionally the High Priestess, acting as leader), because of Wicca's philosophy of balance. There are some female-only covens (as well as men-only covens), but those rules are applicable only to the individual covens in question, and many Wiccans are critical of that approach precisely because of the issue of balance.
Covens certainly exist in real-life as well as here on SOM. I ran a large teaching coven for 12 years and have been part of two other covens.
One often finds people asking whether one should practice as a solitary or as part of a coven. Likewise you hear many opinions on why one way or the other is right or wrong. I contend that there is no one right or wrong way to practice, only what is right for the individual in question.
There are a number of reasons why someone might chose to practice as a solitary instead of with a group. First off there may not be a group that they are aware of in their area. Or they may not have found a group with whom they feel comfortable working
On the one hand, they may just prefer working as a solitary. There are some advantages to this method..you can hold ritual when and where you wish, you can make it as structured or spontaneous as you feel appropriate, you can deal with your issues and needs as you find necessary, you answer to no one but the Goddess and yourself. I know a lot of solitaries who would not trade this freedom of practice for any coven in the world. Heck, even those of us who practice in covens still hold our own solitary rituals to mark the cycle of our daily lives.
On the other hand, there are a bunch of folks out there who like the company of others of like mind, who feel the need for the structure of a group, or who want training that they cannot find in books or in nature. For them, a coven may be the answer. But joining a coven should never be undertaken without some careful forethought and some real research on the group you are considering joining. It pays to learn a bit about the different traditions to find out what you would be most comfortable with. And you have to be really comfortable with the people in the one you chose. Remember that in circle you will drop all your shields and become truly open with you covenmates. That is why the passwords "Perfect Love and Perfect Trust" are so important in a coven setting. Ideally a coven functions somewhat like a family of choice with the members developing close and enduring ties of caring and support. Often this carries over into everyday life with coveners spending much of their time in each other's company. In some ways it is like living as our ancestors did in a clan functioning around a way of living and a living religion. A poorly chosen coven, one with problems with personality conflicts, ethical issues, etc. can be far, far worse than practicing as a solitary. So always look closely at a group for awhile before you commit to joining them.
In short, there is no right or wrong way to be a Witch..you can be one all by yourself...or you can be one with a group. It just depends on you and how you find you best meet your own spiritual needs.
