What is OpenStreetMap?
OpenStreetMap is a map created by people across the world. Everybody can map what is interesting or important for them. It may be nearly anything - from forests through roads and museums to shops. People map buildings, hiking routes, tourism attractions, rocks and fire hydrants.
Collected data can be used by anyone, as long as OpenStreetMap contributors are credited and data stays open. It may be used for making maps, from the usual ones to those showing some specific data like hiking routes. The data was used also in flood preparation planning, making maps for blind people, art projects, in generation of 3D models of cities and in a scientific research. OpenStreetMap also powers routers guiding cars, bicycles, pedestrians and boats.
To be more exact OpenStreetMap is not a map. People are not editing an image, they are recording what exists. For example names of roads, shapes of forests, locations of shops. Map may be created from this data but mappers are not making choices like "this area should be in brown colour to represent a building" or "here should be a blue line to represent a motorway", but rather map things as they exist: "this area is a residential building" or "here is a motorway with 3 lanes in each direction with road code A7".
This data may be presented in many ways - see below for examples. You can select a map style, each style uses the same OpenStreetMap data but displays it differently making it suitable for a specific purpose or audience. Selection below is limited to selection of map styles that are for some reason notable or interesting.
Try to change map styles to see what kind of data is presented in each style. Try to pan around, zoom in and zoom out. Every style has worldwide coverage and is using data from OpenStreetMap database. On small screens list of styles will be collapsed by default, tap layer image in top right to show it and select map style. To hide it again tap on map area.
It is possible to do far more with OpenStreetMap data, see for example some of my recommendations.