Disaster response
Definition: Disaster response refers to the actions taken directly before, during or in the immediate aftermath of a disaster. The objective is to save lives, ensure health and safety and to meet the subsistence needs of the people affected.: 16 
The Business Dictionary provide a more comprehensive definition for "disaster response"; Aggregate of decisions and measures to (1) contain or mitigate the effects of a disastrous event to prevent any further loss of life and/or property, (2) restore order in its immediate aftermath, and (3) re-establish normality through reconstruction and re-rehabilitation shortly thereafter.  The first and immediate response is called emergency response.
The Johns Hopkins and the International Federation of Red Cross and Red Crescent Societies (IFRC) state: "The word disaster implies a sudden overwhelming and unforeseen event. At the household level, a disaster could result in a major illness, death, a substantial economic or social misfortune. At the community level, it could be a flood, a fire, a collapse of buildings in an earthquake, the destruction of livelihoods, an epidemic or displacement through conflict. When occurring at district or provincial level, a large number of people can be affected."
A recent case study of a disaster response undertaken by the IFRC can be viewed here.
The level of disaster response depends on a number of factors and particular situation awareness. Studies undertaken by Son, Aziz and Peña-Mora (2007) shows that "initial work demand gradually spreads and increases based on a wide range of variables including scale of disaster, vulnerability of affected area which in turn is affected by population density, site-specific conditions (e.g. exposure to hazardous conditions) and effects of cascading disasters resulting from inter-dependence between elements of critical infrastructure".
In the British Government's Emergency Response and Recovery guidance, disaster response refers to decisions and actions taken in accordance with the strategic, tactical and operational objectives defined by emergency responders. At a high level these will be to protect life, contain and mitigate the impacts of the emergency and create the conditions for a return to normality. Response encompasses the decisions and actions taken to deal with the immediate effects of an emergency. In many scenarios it is likely to be relatively short and to last for a matter of hours or days—rapid implementation of arrangements for collaboration, co-ordination and communication are, therefore, vital. Response encompasses the effort to deal not only with the direct effects of the emergency itself (e.g. fighting fires, rescuing individuals) but also the indirect effects (e.g. disruption, media interest).
Common objectives for responders are:

saving and protecting human life;
relieving suffering;
containing the emergency – limiting its escalation or spread and mitigating its impacts;
providing the public and businesses with warnings, advice and information;
protecting the health and safety of responding personnel;
safeguarding the environment;
as far as reasonably practicable, protecting property;
maintaining or restoring critical activities;
maintaining normal services at an appropriate level;
promoting and facilitating self-help in affected communities;
facilitating investigations and inquiries (e.g. by preserving the scene and effective records management);
facilitating the recovery of the community (including the humanitarian assistance, economic, infrastructure and environmental impacts);
evaluating the response and recovery effort; and
identifying and taking action to implement lessons identified.

Disaster response planning: The United States National Fire Protection Association (NFPA) 1600 Standard (NFPA, 2010) specify elements of an emergency response, as: defined responsibilities; specific actions to be taken (which must include protective actions for life safety); and communication directives.  Within the standard, NFPA recognize that disasters and day-to-day emergencies are characteristically different.  Nevertheless, the prescribed response elements are the same.
In support of the NFPA standard, Statoil's (2013) practical application of emergency response is across three distinct "lines" that incorporate NFPA's elements. Line 1 is responsible for the operational management of an incident; line 2, typically housed off-site, is responsible for tactical guidance and additional resource management.  Finally, in the case of major incidents, line 3 provides strategic guidance, group resource management, and government and media relations.
While it is impossible to plan for every disaster, crisis or emergency, the Statoil investigation into the terrorist attacks on In Amenas place emphasis on the importance of having a disaster response. The report concludes that a disaster response framework may be utilized in an array of disaster situations, such as that at In Amenas.
Disaster risk reduction (DRR) is action taken to "[reduce] existing disaster risk and [manage] residual risk." DRR plans aim to decrease the amount of disaster response necessary by planning ahead and making communities resilient to any potential hazardous events that might occur. A number of international frameworks such as the Sendai Framework for Disaster Risk Reduction have been enacted to increase the implementation of global mitigation plans in the event of disasters.

Organizations: 

United Nations: The United Nations Office for the Coordination of Humanitarian Affairs (OCHA); is responsible for bringing together humanitarian actors to ensure a coherent response to emergencies that require an international response. OCHA plays a key role in operational coordination in crisis situations. This includes assessing situations and needs; agreeing common priorities; developing common strategies to address issues such as negotiating access, mobilizing funding and other resources; clarifying consistent public messaging; and monitoring progress.

United Kingdom: The organisation in the United Kingdom for the provision of communications disaster response is RAYNET.  The UK organisation for the provision of disaster response by off-road vehicles is 4x4 Response.

European Union: In addition to providing funding to humanitarian aid, the European Commission's Directorate-General for European Civil Protection and Humanitarian Aid Operations (DG-ECHO) is in charge of the EU Civil Protection Mechanism  to coordinate the response to disasters in Europe and beyond and contributes to at least 75% of the transport and/or operational costs of deployments. Established in 2001, the Mechanism fosters cooperation among national civil protection authorities across Europe. Currently 34 countries are members of the Mechanism; all 27 EU Member States in addition to Iceland, Norway, Serbia, North Macedonia, Montenegro, Turkey and Bosnia and Herzegovina. The Mechanism was set up to enable coordinated assistance from the participating states to victims of natural and man-made disasters in Europe and elsewhere.

Canada: In Canada, GlobalMedic was established in 1998 as a non-sectarian humanitarian-aid NGO to provide disaster relief services to large scale catastrophes around the world. Time magazine recognized the work of GlobalMedic in its 2010 Time 100 issue. It has a roster of over 1,000 volunteers from across Canada that includes professional rescuers, police officers, firefighters and paramedics who donate their time to respond to international disasters. Their personnel are divided into Rapid Response Teams (RRTs) that operate rescue units, Water Purification Units (WPUs) designed to provide safe drinking water; and Emergency Medical Units (EMUs) that use inflatable field hospitals to provide emergency medical treatment. Since 2004, GlobalMedic teams have deployed to over 60 humanitarian disasters around the world.

India: In India, the National Disaster Management Authority is responsible for planning for mitigating effects of natural disasters and anticipating and avoiding man-made disasters. It also coordinates the capacity-building and response of government agencies in the time of crises and emergencies. The National Disaster Response Force is an inter-government disaster response agency that specializes in search, rescue and rehabilitation.

United States of America: In the US, the Federal Emergency Management Agency coordinates federal operational and logistical disaster response capability needed to save and sustain lives, minimize suffering, and protect property in a timely and effective manner in communities that become overwhelmed by disasters. The Centers for Disease Control and Prevention offer information for specific types of emergencies, such as disease outbreaks, natural disasters and severe weather, as well as chemical and radiation accidents. Also, the Emergency Preparedness and Response Program of the National Institute for Occupational Safety and Health develops resources to address responder safety and health during responder and recovery operations.
Among volunteers, the American Red Cross is chartered by Congress in 1900 to lead and coordinate non-profit efforts. They are supported by disaster relief organizations from many religious denominations and community service agencies.  Licensed amateur radio operators support most volunteer organizations, and are often affiliated with the American Radio Relay League (ARRL).

Disaster response organizations: In addition to the response by the government, a great deal of assistance in the wake of any disaster comes from charities, disaster response and non-governmental organizations. The biggest international umbrella organizations are the Inter-Agency Standing Committee and the International Council for Voluntary Agencies.
Humanitarian OSM Team works to update and provide map in areas struck by disaster.

Disaster response technologies: 

Ad hoc infrastructure: A range of infrastructures could be restored ad hoc quickly after a disaster using technologies.

Communications: The Government Emergency Telecommunications Service supports federal, state, local and tribal government personnel, industry and non-governmental organizations during a crisis or emergency by providing emergency access and priority handling for local and long-distance calls over the public switched telephone network. There is a Nationwide Wireless Priority Service that allows a user to wait for cellular bandwidth to open.
Wireless mesh networks can be deployed rapidly to enable Internet connectivity, substitute failed mobile phone networks and emergency- and post-disaster communication – including for disaster response coordination and emergency calls. Mesh networks such as B.A.T.M.A.N. are often developed and deployed open-source by volunteer communities with little resources.