At its most basic, an affinity group is a group of people who work together to spread their ideas to the wider public. This can be done though direct actions, propaganda, initiating or working on campaigns, and communicating with popular organisations (such as unions) and communities. It aims not to provide 'leadership' but to give a lead, to act as a catalyst within popular movements.

Autonomous, communal and directly democratic, the group combines revolutionary theory with revolutionary lifestyle in its everyday behaviour. It creates a free space in which revolutionaries can remake themselves individually, and also as social beings.'
-Murray Bookchin, Post-Scarcity Anarchism, p. 221

According to The Spanish Anarchists, affinity groups were created because a 'movement that sought to promote a libratory revolution had to develop libratory and revolutionary forms. This meant that it had to mirror the free society it was trying to achieve not the repressive one it was trying to overthrow. If a movement sought to achieve a world united by solidarity and mutual aid, it had to be guided by these precepts; if it sought to achieve a decentralised, stateless, non-authoritarian society, it had to be structured in accordance with these goals.' (p 180).

The affinity group aims to promote a sense of community and of confidence in ones own abilities, to enable all to be involved in the identification, initiation and management of group/communal needs and decisions. Moreover, it aims to ensure that individuals are in a position (both physicallyas part of a group/community, and mentallyas an individual) to manage their own lives and take direct action in the pursuit of individual and communal needs and desires.

The local affinity group is the means by which a group of individuals coordinate their activities in a community, workplace or social movement. Within these groups, people discuss their ideas, politics, hopes and plans. They write leaflets and organise other propaganda work, and discuss how they are going to work within wider organisations (like the S11 Alliance). There can be affinity groups for different interests and activitiesfor example: to lock down on an entrance to the Crown Casino, to provide medical assistance, perform street theatre or be legal observers.

Affinity groups are most effective when they work as a part of a larger alliance. Such cooperation aims to pool resources and reduce duplicating efforts, expanding the options for the individuals and groups who are part of the alliance. Such an alliance is based upon the 'full autonomy, full independence and therefore full responsibility of individuals and groups.' The alliance of affinity groups is not a central body that decides what people should do, it is a forum of autonomous groups of people who have come together to fight a common enemy.