Solar Wind

The Sun gives of waves of charged particles, called plasma.  These waves begin at incredible speeds, causing a "ring" to form around the solar system.  The Galactic Cosmic Rays generated by the magnetic field of the Milky Way itself, cannot cross into this ring because the output of the Sun's solar wind is so strong.  Galactic Cosmic Rays consist of interstellar medium (the particles that simply float freely in space, mostly Hydrogen and Helium).  The border between where solar wind stops is called the Heliopause.  Inside this boundary is called the Heliosphere.  The point where the solar wind slows to subsonic (under the speed of sound) speeds is called the Termination Shock.  On the other end, the place where Galactic Rays fall below sonic speeds is called the Bow Shock.  Relative to other solar system objects, the Kuiper Belt lies on the edge of the Heliosphere, while the Oort Cloud lies outside it.  The Heliopause is at about 100 AU from the sun (see map below).  No spacecraft has ever passed outside of the Heliosphere to date.


Image from Wikipedia showing a log scale plot of the solar system. For more info on parts of solar system, see Oort Cloud and Kuiper Belt