A drum circle is any group of people playing (usually) hand-drums and percussion in a circle. More recently, however, the term has also been used to designate a particular form of drum circle: a facilitated drum circle.
In this newer definition, a drum circle is a facilitated jam. Drums and/or percussion are handed out or already in place, and people come not to 'learn to drum' but simply to have fun playing together. The facilitator guides and encourages the participants to create exciting in-the-moment music.