NGOs are difficult to define and classify, and the term 'NGO' is not used consistently. As a result, there are many different classifications in use. The most common use a framework that includes orientation and level of operation. An NGO's orientation refers to the type of activities it takes on. These activities might include human rights, environmental, or development work. An NGO's level of operation indicates the scale at which an organization works, such as local, international or national. "Confronting the Classification Problem: Toward a Taxonomy of NGOs" One of the earliest mentions of the acronym "NGO" was in 1945, when the UN was created. The UN, which is an inter-governmental organization, made it possible for certain approved specialized international non-state agencies - or non-governmental organisations - to be awarded observer status at its assemblies and some of its meetings. Later the term became used more widely. Today, according to the UN, any kind of private organization that is independent from government control can be termed an "NGO", provided it is not-profit, non-criminal and not simply an opposition political party. Professor Peter Willetts, from the University of London, argues the definition of NGOs can be interpreted differently by various organizations and depending on a situation’s context. He defines an NGO as “"an independent voluntary association of people acting together on a continuous basis for some common purpose other than achieving government office, making money or illegal activities." In this view, two main types of NGOs are recognized according to the activities they pursue: operational NGOs that deliver services and campaigning NGOs. Although Willetts proposes the operational and campaigning NGOs as a tool to differentiate the main activities of these organizations, he also explains that a single NGO may often be engaged in both activities. Many NGOs also see them as mutually reinforcing.