Peacekeeping
Definitions and types of peacekeeping operations: 

United Nations peacekeeping missions: 

Chapter VI and Chapter VII mission types: There is a range of various types of operations encompassed in peacekeeping. In Page Fortna's book Does Peacekeeping Work?, for instance, she distinguishes four different types of peacekeeping operations. Importantly, these types of missions and how they are conducted are heavily influenced by the mandate in which they are authorized. Three of Fortna's four types are consent-based missions, i.e., so-called "Chapter VI" missions, with the fourth being a "Chapter VII" Mission. 
Chapter VI missions are consent-based; therefore they require the consent of the belligerent factions involved in order to operate. Should they lose that consent, Peacekeepers would be compelled to withdraw. Chapter VII missions, by contrast, do not require consent, though they may have it. If consent is lost at any point, Chapter VII missions would not be required to withdraw.

Observation Missions which consist of small contingents of military or civilian observers tasked with monitoring cease-fires, troop withdrawals, or other conditions outlined in a ceasefire agreement. They are typically unarmed and are primarily tasked with observing and reporting on what is taking place. Thus, they do not possess the capability or mandate to intervene should either side renege on the agreement. Examples of observation missions include UNAVEM II in Angola in 1991 and MINURSO in the Western Sahara.
Interpositional Missions, also known as traditional peacekeeping, are larger contingents of lightly armed troops meant to serve as a buffer between belligerent factions in the aftermath of a conflict. Thus, they serve as a buffer zone between the two sides and can monitor and report on the compliance of either side with regard to parameters established in a given ceasefire agreement. Examples include UNAVEM III in Angola in 1994, and MINUGUA in Guatemala in 1996.
Multidimensional missions are carried out by military and police personnel in which they attempt to implement robust and comprehensive settlements. Not only do they act as observers or in an interpositional role, but they also participate in more multidimensional tasks—such as electoral supervision, police and security forces reform, institution building, economic development, and more. Examples include UNTAG in Namibia, ONUSAL in El Salvador, and ONUMOZ in Mozambique.
Peace enforcement Missions are Chapter VII missions and unlike the previous Chapter VI missions, they do not require the consent of the belligerent parties. These are multidimensional operations comprising both civilian and military personnel. The military force is substantial in size and fairly well-equipped by UN Peacekeeping standards. They are mandated to use force for purposes beyond just self-defence. Examples include ECOMOG and UNAMSIL in West Africa and Sierra Leone in 1999, as well as the NATO operations in Bosnia—IFOR and SFOR.

UN missions during and after the Cold War: During the Cold War, peacekeeping was primarily interpositional in nature—thus being referred to as traditional peacekeeping. UN Peacekeepers were deployed in the aftermath of interstate conflict in order to serve as a buffer between belligerent factions and ensure compliance with the terms of an established peace agreement. Missions were consent-based, and more often than not observers were unarmed—such was the case with UNTSO in the Middle East and UNCIP in India and Pakistan. Others were armed—such as UNEF-I, established during the Suez Crisis. They were largely successful in this role.
In the post-Cold War era, the United Nations has taken on a more nuanced, multidimensional approach to Peacekeeping. In 1992, in the aftermath of the Cold War, then Secretary-General Boutros Boutros-Ghali put together a report detailing his ambitious concepts for the United Nations and Peacekeeping at large. The report, titled An Agenda for Peace, described a multi-faceted and interconnected set of measures he hoped would lead to effective use of the UN in its role in post-Cold War international politics. This included the use of preventative diplomacy, peace-enforcement, peace-making, peace-keeping and post-conflict reconstruction.

Broader aims of UN missions: In The UN Record on Peacekeeping Operations, Michael Doyle and Nicolas Sambanis summarise Boutros Boutros' report as preventative diplomacy, confidence-building measures such as fact-finding missions, observer mandates, and the potential deployment of UN mandated forces as a preventative measure in order to diminish the potential for violence or the danger of violence occurring and thus increasing the prospect for lasting peace. Their definitions are as follows:

Peace-enforcement, meant to act with or without the consent of the belligerents in order to ensure any treaty or cease-fire mandated by the United Nations Security Council is maintained. This is done primarily under the auspices of Chapter VII of the UN Charter and the forces are generally heavily armed as opposed to the unarmed, or lightly-armed personnel frequently deployed as observers.
Peace-making, meant to compel belligerents to seek a peaceful settlement for their differences via mediation and other forms of negotiation provided by the UN under the auspices of Chapter VI of the UN Charter.
Peace-keeping, deployment of a lightly-armed United Nations presence in the field with the consent of the belligerents involved in order to build confidence and monitor any agreements between concerned parties. Additionally, diplomats would continue to work toward comprehensive and lasting peace, or for the implementation of an agreed-upon peace.
Post-Conflict Reconstruction, intended to develop economic and social cooperation meant to mend relations between the belligerents. Social, political, and economic infrastructure would ideally prevent potential violence and conflict in the future and help to contribute to lasting and robust peace.
Peacekeeping also means working together with NGOs with a view to protecting cultural property. The UN peacekeeping commitment to the protection of cultural heritage dates back to 2012 and is being expanded. An outstanding mission was the deployment of the UN peace mission UNIFIL together with Blue Shield International in 2019 to protect the UNESCO World Heritage in Lebanon. Basically, the protection of cultural property (carried out by military and civil experts in cooperation with local people) forms the stable basis for the future peaceful and economic development of a city, region or country in many conflict areas. Whereby there is also a connection between cultural user disruption and the cause of flight, as President of Blue Shield International Karl von Habsburg explained during the United Nations peacekeeping and UNESCO mission in Lebanon in April 2019: "Cultural assets are part of the identity of the people who live in a certain place. If you destroy their culture, you also destroy their identity. Many people are uprooted, often have no prospects anymore and subsequently flee from their homeland".

Non-United Nations peacekeeping: Not all international peacekeeping forces have been directly controlled by the United Nations. In 1981, an agreement between Israel and Egypt formed the Multinational Force and Observers, which continues to monitor the Sinai Peninsula.
The African Union (AU) is working on building an African Peace and Security Architecture that fulfils the mandate to enforce peace and security on the continent. In cases of genocide or other serious human rights violations, an AU-mission could be launched even against the wishes of the government of the country concerned, as long as it is approved by the AU General Assembly. The establishment of the African Peace and Security Architecture (APSA) which includes the African Standby Force (ASF) is planned earliest for 2015. On the regional level, the Economic Community of West African States has initiated several peacekeeping missions in some of its member states, and it has been described as "Africa's most advanced regional peace and security mechanism".
Unarmed Civilian Peacekeeping (UCP) are civilian personnel that carry out non-violent, non-interventionist and impartial set of tactics in order to protect civilians in conflict zones from violence in addition to supporting additional efforts to build a lasting peace. While the term UCP is not entirely ubiquitous among non-governmental agencies (NGOs) in the field: many utilize similar techniques and desire shared outcomes for peace; such as accompaniment, presence, rumour control, community security meetings, the securing of safe passage, and monitoring.

Brief history: 

Creation and early years: United Nations Peacekeeping started in 1948 when the United Nations Security Council authorised the deployment of UN unarmed military observers to the Middle East in order to monitor the armistice agreement that was signed between Israel and its Arab neighbours in the wake of the Arab-Israeli War. This operation was called the United Nations Truce Supervision Organization (UNTSO) and is still in operation today. With the passage of resolution 73 (1949) by the Security Council in August 1949, UNTSO was given the task of fulfilling four Armistice Agreements between the state of Israel and the Arab states which had participated in the war. Thus, UNTSO's operations were spread through five states in the region—Israel, Egypt, Jordan, Lebanon and the Syrian Arab Republic.

Cold War peacekeeping: In the wake of independence in India and Pakistan in August 1947 and the subsequent bloodshed that followed the Security Council adopted resolution 39 (1948) in January 1948 in order to create the United Nations Commission for India and Pakistan (UNCIP), with the purpose of mediating the dispute between India and Pakistan over Kashmir and the fighting related to it. This operation was non-interventionist in nature and was additionally tasked with supervision of a ceasefire signed by Pakistan and India in the state of Jammu and Kashmir.