Just as many Wiccans are steering clear of the term "witch", many are steering clear of the term "coven". Due to the negative light that the Inquisition, fundamental Christianity and society in general has put on the words, many Wiccans of all traditions are opting to avoid these terms that carry alot of false information and prejudice. Slander and spiritual hatred is never attractive.
A Circle is often a group of solitary practitioners of varying beliefs who gather to celebrate Esbats, Sabbats, and special healing or prosperity rituals with total respect to each other's personal beliefs and views of The Divine. Usually this type of eclectic Circle or a group of Wiccans of various traditions does not have an appointed Priest or Priestess.
Instead, there is usually a group facilitator of the rituals; he/she organizes the parts to which each person is going to take which part of the ceremony. For example, someone is appointed to cast the protective circle, one is appointed to call the quadrants, one is appointed to invoke The Divine Spirit, and others are appointed for coning the power towards the goal of the ritual, as well as the other components of the ceremony.
A Coven is a more organized group. There is usually only one tradition observed among all members of the Coven, and the belief structure is very similar, if not nearly identical between the practitioners. In addition, there is usually a hierarchy of leaders within the Sacred Space. Christians should be able to understand this as having a Pastor, Elders and Deacons, or Priests, Ministers, Lay Ministers, Reverends, Stewards, Music Directors, and Youth Leaders. This is the same for Wiccan Covens.
Covens and Circles can be all male, all female or both male and female. This is a personal choice of each group regardless of being a Coven or a Circle. Many groups feel that male and female personal energies do not work together when raising the cone of power towards a particular goal; other groups feel no problems in mixing personal energies of the two sexes.
Sometimes there are problems of mismatched energies in any Circle and Coven, regardless if the group of practitioners is the same-sex or mixed. Often, not everyone's goals and intentions are the same; some have a less positive outlook on life and this can affect the entire group.