Image credit: NASA
The locations of Voyager 1 and Voyager 2 as of 2005. The termination shock is the boundary marking one of the outer limits of the sun's influence. It is where the bubble of solar wind particles slows down to subsonic speed (with respect to the star) due to interactions with the local interstellar medium. This causes compression, heating, and a change in the magnetic field. Beyond this is the heliosheath, which ends at the heliopause - the boundary where the Sun's solar wind is stopped by the interstellar medium. This is not spherical as the sun is moving around the center of the galaxy; in this image, it is moving to the left. This sets up a bow shock in the interstellar medium.