Orientation

Low polar orbits are so named because satellites using them circumnavigate the planet in a north-south orientation, passing near or over the North and South Poles.

How High?

While a low orbit may be defined as occurring anywhere from 200 to 900 miles above the Earth, most planned, lengthy low orbits take place at a distance of at least 600 miles from the surface to avoid undue atmospheric friction.

Escape Velocity

Objects circling in a low polar orbit must be moving at a speed of at least 17,000 mph to overcome Earth's gravitational field. At this speed, it takes an object between 90 and 100 minutes to complete each circumnavigation.

Uses

Because the rotation of the Earth allows satellites to observe the greatest portion of the planet as they circle, low polar orbits are often used to study features on its surface, plot navigation, learn about weather patterns, conduct military surveillance and broadcast communication signals.

Fun Fact

According to satellite information website Orbital, a New York Times article said the U.S. government keeps track of roughly 10,000 pieces of debris orbiting the Earth. Most of these objects are in low orbits.

Source:

Nasa.gov: Polar Orbiting Satellites

Satellite Orbits: Low Earth Orbit