We discuss the two main types of organization that seek to influence or control national policy: political parties and organized groups (frequently referred to as pressure or interest groups).
Parties are usually distinguished from groups by their intention to seek control of the main offices of government.
They therefore put forward a wide spectrum of policies to cover all politically significant aspects of national life, as well as foreign affairs.
Organized groups, on the other hand, restrict their aims to those that will promote the interests of their members or further particular causes; they do not have a comprehensive political programme, nor do they seek to acquire control of the main offices of government.
Their purpose is to influence government to adopt policies favourable to them.

Some difficulties arise, however, from the generally accepted distinction.
Not all organizations that claim to be seeking control of the government can realistically be said to be doing so.
Parties such as Plaid Cymru or the Green Party in Great Britain have no chance of controlling the government, although they may get members of parliament (MPs) elected and, in certain circumstances, have some say in making the government's life easier or more difficult.
At the very most, they can hope for a modest share in a possible coalition government.
In other words, for practical purposes their aim must be to influence the government rather than to control it.