The Voyager 1 spacecraft, launched in 1977 to explore the planets, is now agreed by scientists to have entered the heliosheath at the edge of the solar system 8.7 billion miles (14 billion kilometers) from the Sun.  In a few years, Voyager 1 is expected to become the first man-made object to cross into interstellar space.
"Voyager has entered the final lap on its race to the edge of interstellar space, as it begins exploring the solar system's final frontier," said Dr. Edward Stone, Voyager project scientist at the California Institute of Technology.
As long ago as 2003, scientists thought Voyager 1 was entering the termination shock region of the solar system, but there was some dispute.  The termination shock is the area preceding the heliosheath, where the electrically charged solar wind is slowed and concentrated by contact with interstellar gas.
The heliosheath is considered the outer edge of our solar system.  Around it is the heliopause, a cosmic bubble where the pressure of solar wind and interstellar wind is in balance.  
The solar system as a whole is in orbit around the center of the Milky Way galaxy.  As it plows through clouds of interstellar gas and dust, a bow shock forms ahead of it, which has been compared to the turbulence a ship creates as it sails through ocean currents.  All this is illustrated in the NASA diagram shown here.
Voyager 1 is still operational and sending back reams of scientific data.  Already notable for more than 27 years of successful operation, Voyager 1 is projected to continue operating on its plutonium power source possibly until the year 2020.
Voyager 2, its companion probe launched the same year, has visited more planets than any other spacecraft.  It too is eventually expected to exit the solar system at a more downward angle, but is currently only 80% as far from the sun as Voyager 1.
The Voyagers are not only gathering data about the cosmos, they are sending the greetings of the human world out to it.  Each Voyager contains what is known as the "Voyager Golden Record", which is an audio recording on a 12-inch gold plated copper disc.  The discs contain samples of nature sounds, spoken greetings in 55 human languages, and musical compositions.  
The collection of samples was supervised by astronomer Carl Sagan, and is intended to convey the hopefulness of life on Earth for possible discovery by alien races or retrieval by some advanced human civilization.

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Voyager 1 enters heliosheath at edge of solar system