Usually a group or panel interview includes two or more interviewers. These interviews are designed to provide multiple perspectives about a prospective employee. Often you will be watched to see how well you handle multiple perspectives and personalities.
Questions are similar to those asked in a Technical Interview. However, interviewers take turns asking the questions. It is important to be relaxed and thoughtful, direct your answers in particular to the person who asked the question but be sure to make eye contact with each person in the group.
Additionally, a group interview might mean that you are being interviewed as part of a group: employers may convene a number of prospective employees and put them through the interview process simultaneously. Group interviews, in this sense, are designed to ascertain how you might conduct yourself in a team based working environment.
There may be a question period similar to the interview styles described, you may be asked to complete a project with some of your fellow interviewees, and/or you may be asked to complete a written test. A Group interview of this nature can be used as a screening procedure and may occur as part of an interview series that includes Phone, Technical, Panel, and HR interviews over a number of appointments.