Immediately after the massive 7.8 magnitude earthquake that hit Nepal on Saturday, 25th April 2015, the Humanitarian OpenStreetMap Team [HOT] activated a huge user-contributed effort towards mapping various areas of the affected region. Along with organizations like Kathmandu Living Labs and Mapbox, HOT has enabled 3,461 users all over the world to contribute to OpenStreetMap [OSM] of Nepal till now. Around 57,000 edits have been made in the past four days by users, 2061 of whom are new users, registering just for this task . These maps help relief and rescue organizations on the ground to reach affected areas and citizens.
Facilitating a link between the OpenStreetMap Community and the traditional crisis / disaster response teams, has been the main role of HOT. They have helped setup voluntary user contributions and mapping tasks during disasters like the Haiti earthquake, Ivory Coast Election Crisis, Ebola Epidemic in West Africa and Somalian Drought & Famine Crisis. Kathmandu Living Labs is a group of young technology enthusiasts who have remarkably brought together open source, open data and civic technologies to solve difficult issues. Along with Mapbox, an agency that specializes in maps, spatial data and Geographical Information Systems, they are actively working on HOT OSM tasks.
Since Sunday, 26th April 2015, users have been able to map in great detail about 10,000 sq. km of the affected area, “including coverage of road networks, hiking trails, built-up areas, building footprints, river crossings and temporary relief camps.” 
How are maps useful for relief and rescue work?
Maps are valuable tools for humanitarian work. They allow for a strategic understanding of different parts of affected regions, such as road networks, residential areas, types of land use (industrial, agricultural) and so on. This helps relief and rescue agencies to identify all locations, how to reach them and what kind of a topographic landscape to plan for. So in a natural disaster like an earthquake, one of the many ways a detailed map can help is to identify locations where camp sites (Internally Displaced Persons – IDP Camps) can be setup. Mapping a network of roads, paths and tracks is another very useful task, since the data it generates is highly valuable for distributing aid and relief services.
Organizations like Red Cross and MSF (Doctors Without Borders) work closely with HOT for maps.
What is OpenStreetMap and how is it different from Google Maps?
OpenStreetMap, or OSM, is like a Wikipedia of maps – all data on these maps is user contributed. And unlike Google Maps, this data is freely available for download, distribution, adaption and use. This means that such map data can be used offline and for particular needs of different situations. You can create a map of power lines running through a country to see the reach of the electrical grid, if ever the need arises.
This makes OSM indispensable for humanitarian work. Map data of Nepal contributed by users is available here and here – these are updated every 30 minutes. This can be used to print physical maps, used on smart phone apps like OSM that work offline and even support voice navigation, and on GARMIN GPS devices. 
Another version published on Youth Ki Awaaz.