Along for the ride is a "message in a bottle"--a gold-plated phonograph record containing greetings from earth--waiting to be found by some alien species.
As Ann Druyan, science media producer and widow of Carl Sagan, put it:
“This was the most romantic and beautiful project ever attempted by NASA. It
had the sounds of a kiss, a mother saying hello to her newborn baby for the first time, all that glorious music. Remember, this was during the Cold War. Everyone was living with the knowledge that 50,000 nuclear weapons could go off at any time, and there was a lot of angst about the future. This was something positive -- a way to represent Earth and put our best foot forward. That was irresistible.”
NASA, for its part, was just glad the record didn't have any naked people on it.
Anyway, no extraterrestrials have picked up the record yet, but it has become a pop culture icon, figuring into a Saturday Night Live sketch, one of The Strokes' videos, episodes of The West Wing and The X-Files, and apparently driving the plot of the Transformers Beast Wars show. (The nice people at Wikipedia, of course, maintain an exhuastive record of the disk's appearances in fiction.)
Probably not what Carl Sagan and friends had in mind. Or maybe it's exactly what they wanted--not to bring a message about Earth to space, but to bring a message about Earth to Earth.