What kind of orbit are observation satellites usually placed in?

You mean Earth observation satellites, and the answer is near-Polar low-Earth orbits.

This means that as the satellite orbits from above one pole to the other, the Earth turns beneath it, and the satellite passes over the Equator, and every other point on its orbit, at a different place on each successive orbit. In this way, after enough orbits, it can view the whole Earth.

Earth observation satellites are placed in low-Earth orbit because (a) they travel faster over the ground at lower altitude and (b) being closer to the ground, their telescopes do not need to be so powerful to achieve a given spatial resolution.

Military observation, or spy, satellites, are often placed in a high parking orbit until they are needed, whereupon they are placed into a highly elliptical orbit to allow them to come very close to Earth (and hence see it with great detail) over the region of interest.