Group dynamics refers to the attitudinal and behavioral characteristics of a group. Group dynamics concern how groups form, their structure and process, and how they function. As applied to group development, group dynamics is concerned with why and how groups develop. There are several theories as to why groups develop. According to Tuckman's theory, there are five stages of group development: forming, storming, norming, performing, and adjourning. During these stages group members must address several issues and the way in which these issues are resolved determines whether the group will succeed in accomplishing its tasks. One common way to classify group is by whether they are formal or informal in nature. Formal work groups are established by an organization to achieve organizational goals. Formal groups may take the form of command groups, task groups, and functional groups. Group structure is a pattern of relationships among members that hold the group together and help it achieve assigned goals. Structure can be described in a variety of ways. Among the more common considerations are group size, group roles, group norms, and group cohesiveness.