In individual therapy, a patient meets one-on-one with a trained psychologist or counselor. Therapy is a pivotal part of effective substance abuse treatment, as it often covers root causes of addiction, including challenges faced by the patient in their social, family, and work/school life.
Group therapy is any therapeutic work that happens in a group (not one-on-one). There are a number of different group therapy modalities, including support groups, experiential therapy, psycho-education, and more. Group therapy involves treatment as well as processing interaction between group members.