This is the area around the Earth, that is dominated by the planets' magnetic field. This field, protects life on Earth from solar and other cosmic particle radiation, and from erosion by the solar wind - the constant flow of charged particles flowing from the Sun - which would otherwise strip away the Earth's upper atmosphere, including the ozone layer.
It is believed that it is generated by the convective motion of charged, molten iron, within Earth's outer core. It's shape is formed by the constant bombardment of the solar winds from the side facing the Sun (the 'day-side'). The 'night-side' extends out into a long tail, well beyond the moon. The Magnetosphere is a dynamic, interconnected system, responding to the Sun, planetary, and interstellar flux.
The images, except the last in the sequence (The Magnetic Poles and the Geodynamo) show the Sun, the Earth and the Moon. The Earth and Moon size and distances in between are to scale, but are about 20 times larger in proportion to the Sun, and are much closer.
To find out more about the Earth's Magnetosphere, and magnetism, then browse the blog posts below.