A pressure group is an organisation with shared aims which seeks to influence policy through political means without seeking political office itself.
Robert Dahl theorised pluralism which suggests power is dispersed equally between multiple groups in society so that the majority opinion will prevail. Smaller pressure groups of a similar nature can combine to have similar influence to larger ones.
C. Wright Mills theorised elitism which suggest power is held by a small group of elites, and influence is only achievable for those with status and money. This points to the discrepancy between insider and outsider pressure groups.
Interest/sectional groups only represent a fraction of the population e.g. trade unions like the NEU and professional associations like the BMA. This can mean their interests contradict those of the rest of the UK.
Cause/promotional groups aim to bring education on a cause relevant to everyone e.g. RSPCA, environmentalist groups. As a result they may be less likely to directly influence specific policy as to build a political agenda.
Pressure groups allow people to be represented, creating a link between the people and the government e.g. Stonewall represents queer people. However leaders are usually unelected.
Pressure groups allow more direct, active and frequent participation. However not all who join pressure groups will join them to participate e.g. National Trust.
Programme monitoring is a function of pressure groups, for example Ban Conversion Therapy does this. However this does not mean they will necessarily influence policy.
Pressure groups' status can change over time. While trade unions and religious groups have heavily decreased in influence, environmental groups have increased in influence.
Public education and political agenda buildinga are also functions of pressure groups.
People may join pressure groups for participation, because of their personal beliefs or for representation.
People may join pressure groups if it is a need, e.g. being part of a trade union, or equally for material benefits e.g. National Trust.
Pressure groups are significantly more subscribed than UK political parties; the RSPB has a larger membership than all the parties combined at 2m members and the National Trust is the largest pressure group in Europe at 3.4m.