A pressure group is an organization that seeks to influence government decision makers (public policy) and or protect or advance a particular cause or interest. They often use websites and the media to educate people/spread propaganda. Groups campaign a specific issue and raise it up the political agenda. Campaigning raises funds for and inevitably resources for that cause.

Outsider groups are those whose aims do not enjoy/ have the support of the Government. They tend to adopt media based, high profile campaigns to capture public attention. E.g. Animal Liberation Front.
Insider groups have the support and attention of Government. Government will often approach them for advice. They will often have long standing and regular discussions with Government. A good example here is the work of Scottish members of the British Medical Association.
Sectional groups represent and defend the interests of particular sections of society - often economic interests, but also consumer and professional interests. E.g. Trade union.
Promotional groups are usually formed for a specific purpose - to promote a particular cause or campaign on a particular issue. In this sense, they have no other function apart from their role as pressure groups E.g. Green peace.
Ad hoc groups are formed on temporary issues and tend to be on a temporary basis. The group disband once their cause has disappeared. Pressure groups often experience division amongst their members, and can therefore end up as merely temporary groups (e.g. Fathers 4 Justice, which was disbanded in 2006).
Permanent groups set up in order to campaign for an ongoing cause E.g. the National Union of Students

The reasons for why some pressure groups are more successful than others is due to their relationship with the government. Some insider pressure groups are in close contact with senior civil servants and ministers and are therefore able to influence legislation. For example, Trade Unions and the British Medical Association may be asked to share their expertise and policy details. These groups may be more successful than ‘outside’ groups, who will find it hard to influence government legislation. These may be groups who wish o work out with the government such as Greenpeace or because of their violent methods cannot be associated with the government, for example Anti Animal Testing Groups. Therefore, these groups may be less successful in directly influencing the government.