As the solar wind streams out of the solar system it eventually runs into the interstellar medium. The heliosphere, the region where the solar wind and the interplanetary magnetic field pulled out of the Sun by the solar wind are unaffected by the interstellar medium, consists of a bubble-shaped region surrounding the Sun. In the direction the Sun moves through the interstellar medium a bow shock is formed similar to that in between the magnetic field of the Earth and the interplanetary magnetic field; and in between the bow shock and the heliosphere is the heliosheath, a region where the effects of the interstellar medium are mingled with that of the heliosphere. The image below shows the geometry involved, and should be compared to the image of The Magnetic Field of the Earth. (Walt Feimer, NASA)

The Edge Of The Solar System
     As can be seen in the image above the Voyager spacecraft happened to be headed in the direction of the leading edge of the heliosphere. As they neared the edge of the heliosphere and the bow shock created by its interaction with interstellar gases, the Solar Wind slowed and stopped (and to a certain extent rebounded toward the Sun). During a roughly decade-long period starting near 2005, fluctuations in the activity of the Sun pushed the leading edge of the heliosphere outward, then allowed it to be pushed inward, so that sometimes the Voyagers were closer to the boundary between the region controlled by the Solar Wind and the region controlled by interstellar gases, and sometimes they were further from that boundary. But between 2012 and 2015 they definitely crossed the boundary and found themselves in a region dominated by interstellar gases. Since that time there have been numerous news articles about how the spacecraft have left the Solar System, and are now in interstellar space. However, that is misleading. What those articles should really say is that the interstellar medium permeates the vast majority of the outer Solar System, and only a small region close to the Sun (within about 100 AUs of the Sun in the leading direction) is dominated by the Solar Wind. The actual boundary of the Solar System is determined by how far out solid bodies, such as those in the Kuiper Disk (which extends outwards to perhaps 2000 AUs from the Sun) and the Oort Cloud (which extends outwards to between 30 and 50 thousand AUs from the Sun), are in orbit around the Sun and have motions controlled more by its gravity than by any other object. Since this represents a region 300 to 500 times further from the Sun than the heliospheric boundary, although the Voyagers are now outside the heliosphere, they are still far closer to the Sun than they are to the outer limits of the Solar System, and even at their approximately 16 km/sec speed, it will take them several thousand years to actually leave the Solar System, and about 40 thousand years to reach a point (a little over 2 light years away) about half as far from the Sun as the nearest known stars.