The message was created for the dedication of the Arecibo Observatory in Puerto Rico, which remains the most powerful radio telescope in the world. (It was also used as the filming location for the climax of GoldenEye, so really it's just chock full of feathers in its metaphorical cap.) To see the message more clearly, click on the image up top to expand.
Designed by Frank Drake - the inventor of the eponymous Drake equation, which estimates the number of alien civilizations in the universe - with the help of Carl Sagan and other luminaries, the message was beamed at a frequency of 2380 megahertz at a power of 1000 kilowatts towards the globular cluster M13, which was one of the nearest densely packed parts of the sky available when the observatory was dedicated.
The message is just 210 bytes of information organized onto a grid of 1679 binary digits. That particular number was chosen because it's semiprime, meaning it is the product of two prime numbers, in this case 23 and 73. That means any civilization that received the message would know the grid would have to be 23 by 73, and only one of the two possible arrangements presents intelligible information. NASA provides a rundown of what's on the message:
The above message gives a few simple facts about humanity and its knowledge: from left to right are numbers from one to ten, atoms including hydrogen and carbon, some interesting molecules, DNA, a human with description, basics of our Solar System, and basics of the sending telescope.
Of course, we can't expect to hear anything back in even the remotely near future - since M13 is 25,000 light-years away, the earliest we'll be hearing back from any alien civilizations is the year 51974...give or take a millennium or two.