The solar wind is a constant stream of loose electrons and positrons (beta rays), as well as other charged particles, all of which originate from the Sun. This is distinct from sunshine, which consists of electromagnetic radiation.
What is the solar wind made of?
Beta rays make up nearly all of the solar wind. The next most common component of the solar wind are lone protons, or hydrogen nuclei which have been stripped of all electrons. A few alpha particles, or stripped helium nuclei, are also included in the solar wind.
All of these particles are streaming outwards from the Sun at speeds of up to 540 miles per second. Although they are extremely hot, with temperatures higher than a million degrees, the bitterly cold background temperature of space means that the heat from these charged particles is constantly radiated away.
How the solar wind impacts the Earth
The charged particles of the solar wind interact with the Earth's magnetic field. The strength of the solar wind bends the magnetic lines away from the Sun, so that the field lines closest to the Sun are compressed inwards towards the Earth and the field lines furthest away from the Sun are extended outwards, away from the Earth.
When the charged particles are caught by the Earth's magnetic field, the field lines deflect them towards the polar regions. This is what causes the aurora borealis and australis. When nitrogen, oxygen, and other atmospheric gases lose or regain electrons, they give off photons which show up as visible light.
Usually the magnetic field catches and deflects charged particles without much external sign of this crucial activity except these polar auroras. Outside the Earth, the solar wind also causes the tails of comets to point away from the Sun.
Extreme effects of the solar wind
During high intensity solar flares and coronal mass ejections (CME), the charged particles of the solar wind can compress the "windward" side of Earth's magnetic field to the point that geosynchronous satellites are no longer protected by it. Uneven bombardment of ions can cause differential charges and arcing, which can damage communications and GPS satellites. Because of the expansion of the atmosphere when heated by increased radiation, Low-Earth Orbit satellites experience greater atmospheric friction and may fall out of orbit faster than expected.
On rare occasions, so much solar radiation is released that the Earth's magnetosphere may be overwhelmed. When this happens, the same charged solar particles which are responsible for the aurora borealis can also induce currents in hydro and cellular towers, which can damage or destroy them. This can disrupt the electrical grid or cell network, possibly for a long time. There may also be radio wave interference, but that is caused by sunshine, not by the solar wind.
An equivalent event has happened before, on a much smaller scale than would be the case today. On September 1 and 2, 1859, the Earth was bombarded by the strongest confirmed solar storm in human history. When the solar particles from the flare reached Earth, they were in opposite alignment with the Earth's magnetic field and were strong enough to overwhelm it.
As a result, auroras were visible as far south as Hawai'i. In Boston, the aurora was "so brilliant that about one o'clock ordinary print could be read by [its] light." The auroral current was also strong enough for a few telegraph operators to cut off their battery power and communicate using only the geomagnetically induced current. However, this was rare. In most cases, the auroral currents shorted out telegraph wires and even caused fires.