It is a matter of common experience that when an object is projected upwards from the Earth, its upward speed decreases, reaches zero, and then reverses as gravity inevitably has its way and accelerates the object towards the ground again.
Henry Wadsworth Longfellow had an example: "I shot an arrow in the air; it fell to earth I knew not where."
But the sequence is not as inevitable as everyday experience might suggest. If the upward speed of a body exceeds a certain threshold of about seven miles per second, the so-called "escape velocity", gravity is overcome. Jules Verne provides an example here.
In an adventure story written in 1865 and called From the Earth to the Moon, he has the three heroes launched from an enormous cannon from a spot in Florida uncannily close to Cape Canaveral; rather than returning to Earth, they are trapped in space, destined forever to circumnavigate the universe.
In between these two extremes, however, is a situation where an object does not return to Earth but ultimately reaches a state of equilibrium which, in ideal conditions, would allow it to circle the world forever as an artificial satellite. This idyllic condition depends upon an exact balance between the gravitational pull of the Earth on the one hand, and the centrifugal forces acting on a body moving in a circle on the other.
An object orbiting in this fashion resembles a conker whirling on a length of string, except that in the case of a satellite the restraint provided by the string is replaced by the invisible tug of the Earth's gravity.
Depending on its height, a precise orbital speed is required to maintain a satellite in this delicate state of balance. In general, the lower the satellite, the stronger the force of gravity, and the faster the satellite must whiz around. If a spacecraft moves too slowly, gravity wins the tug-o'-war, and it falls to Earth again: if it is moving too fast for a particular orbit, the more powerful centrifugal force will cause a satellite to drift farther from the Earth, and the required balance may be achieved at some higher altitude.
Another complication enters the picture if a satellite is in a very low orbit. The Earth's atmosphere is then sufficiently dense to result in a significant amount of drag; it causes the satellite to slow down, and therefore to be drawn even closer to the ground - and into even thicker air with each succeeding revolution. It is ultimately burned up by atmospheric friction.