Earth observation satellites are satellites specifically designed to observe Earth from orbit, similar to spy satellites but intended for non-military uses such as environmental monitoring, meteorology, map making etc.
Most Earth observation satellites carry instruments that should be operated at a relatively low altitude. Altitudes below 500-600 kilometers are in general avoided, though, because of the significant air-drag at such low altitudes making frequent orbit raising manoeuvres necessary. The Earth observation satellites ERS-1, ERS-2 and Envisat of European Space Agency as well as the MetOp spacecraft of the European Organisation for the Exploitation of Meteorological Satellites are all operated at altitudes of about 800 km. The Proba-1, Proba-2 and SMOS spacecraft of European Space Agency are observing the Earth from an altitude of about 700 km.
To get a (close to) global coverage with such a relatively low orbit it has to be polar or at least close to polar. As such a rather low orbit will have an orbital period of roughly 100 minutes the Earth will rotate around its polar axis with about 25 deg between successive orbits with the result that the ground track is shifted towards west with these 25 deg in longitude.
For spacecraft carrying instruments for which an altitude of 36000 km is suitable the Geostationary orbit is sometimes the preferred choice. From such an orbit one gets uninterrupted coverage of more than 1/3 of the Earth. With 3 geostationary spacecraft positioned over the equator at longitudes separated with 120 deg the whole Earth is covered except the extreme polar regions. This type of orbit is mainly used for meteorological satellites.