Pressure groups seek to influence policy or public bodies. Usually they arise as the result of a social movement.
A pressure group is an organization which seeks as one of its functions to influence the formulation and implementation of public policy’ (Grant 2000) or
‘The field of organized groups possessing both formal structure and real common interests in so far as they influence the decisions of public bodies’ (W J M Mackenzie)
Unlike social movements, pressure groups usually have a single issue. Pressure groups are also more likely to be organised (formally) and have (registered) members. Social movements, on the other hand, tend to have 'supporters'.
Examples of social movements include the women's and youth movements. These aim to change, or raise awareness, of the social order. Out of these groups, particular issues may arise that require political (rather than social) change. Pressure groups then form from especially passionate members of the social group. These pressure groups would then use political strategies in order to gain new or modified legislation on one issue.
In this way, social groups (i.e. Amnesty International), might support, provide members or resources for, several pressure groups. Each targeting a specific government on a specific issue.