The solar wind is a stream of charged particles released from the upper atmosphere of the Sun. It mostly consists of electrons and protons. The source of the solar wind is the Sun's hot corona. The temperature of the corona is so high that the Sun's gravity cannot hold on to it. The solar wind speed varies between 300 km/s and 800 km/s. These high and low speed streams interact with each other and alternately pass by the Earth as the Sun rotates.
As the solar wind approaches a planet that has a well-developed magnetic field, e.g. the Earth, Jupiter and Saturn, the particles are deflected by the Lorentz force. This region is known as the magnetosphere and it causes the particles to travel around the planet rather than bombarding the atmosphere or surface. The magnetosphere is roughly shaped like a hemisphere on the side facing the Sun, then is drawn out in a long wake on the opposite side. Earth itself is largely protected from the solar wind by its magnetic field, which deflects most of the charged particles. A smaller number of particles from the solar wind manage to travel to the Earth's upper atmosphere and ionosphere in the auroral zones. That produces phenomena such as the aurora and geomagnetic storms.
Image source: weirwarpd
Astral bodies without magnetic field, e.g. the comets, are not able to deflect the solar wind; therefore, they have long tails pointing opposite to the sun. Venus, the nearest and most similar planet to Earth in the Solar System, has an atmosphere 100 times denser than our own, with little or no geo-magnetic field; hence, ia has a comet-like tail that extends to the orbit of the Earth.
Image source: The Solar System lcgot.net
Additional reading:
Solar Physics: The Solar Wind
Solar Wind in Wikipedia
