Pressure groups are organisations that wish to influence political decision making. They do not wish to hold power for themselves but rather persuade decision makers such as MPs who do.
In the UK, there are a number of well-established, well-funded, and well-organised pressure groups that have been very effective in influencing decision-makers.
These have open membership from the public and promote a cause. For example, Friends of the Earth, which is concerned with protecting the environment.
These are only open to certain individuals, like the members of a trade union. For example, the British Medical Association is only open to doctors. They campaign on issues affecting the medical profession, such as the recent dispute over contract proposals.
These have close links with the government. They will give advice and will be consulted prior to legislation which may affect that group.
These types of pressure group are more likely to be successful. For example, the CBI (the Confederation of British Industry) are regularly consulted on policy by the government and the government worked closely with the CBI when arguing for the UK to remain in the European Union.
In Scotland there are a number of well-established 'insider' pressure groups such as Shelter Scotland which campaigns on behalf of people with poor housing and the homeless in Scotland.
These groups often take action of which the government often disapproves. Sometimes outsider groups often engage in civil disobedience or direct action in order to reinforce their point. Some outsider groups are also wealthy and use a great deal of publicity to attract people to promote their cause.
Greenpeace Scotland and Friends of the Earth Scotland are both examples of 'outsider' pressure groups. Recently both have organised a series of protests because they are opposed to the use of fracking to obtain gas from underground deposits.