Short answers, pulled from the story.
An Earth observation satellite is used to observe Earth from orbit for purposes such as environmental monitoring, meteorology, cartography, and, in some cases, military spying. The most common type takes satellite images similar to aerial photographs, though some perform remote sensing without producing pictures, as with GNSS radio occultation.
Satellite remote sensing dates to the launch of Sputnik 1 by the Soviet Union on the 4th of October 1957, which sent back radio signals that scientists used to study the ionosphere. The first American satellite, Explorer 1, followed on the 31st of January 1958 and led to the discovery of the Van Allen radiation belts.
In 2008, more than 150 Earth observation satellites were in orbit, gathering over ten terabits of data daily. By 2021 that number had grown to over 950, with the US company Planet Labs operating the largest number of them.
Most Earth observation satellites orbit above 500-600 kilometers to avoid the air drag found at lower altitudes. Some, such as ERS-1, ERS-2, Envisat, and MetOp, orbit at about 800 kilometers, while geostationary weather satellites orbit much higher, at about 36,000 kilometers.
A polar orbit lets a satellite achieve global coverage, since the Earth rotates about 25 degrees beneath it between successive orbits, shifting its ground track west each pass. Sun-synchronous orbits pass over each spot on the ground at the same time of day, making observations from different passes easier to compare.
Herman Potočnik explored the idea of using orbiting spacecraft for peaceful and military observation of the ground in his 1928 book, The Problem of Space Travel. The book also described geostationary satellites, an idea first put forward by Konstantin Tsiolkovsky.