In case you haven't "turned on" Google's homepage, it's basically celebrating Robert Moog's birthday with a fully-functioning analogue synth attached to an old-fashioned reel-to-reel tape machine. OH MY GOD, this is incredible. There goes my day.
Bob Moog (b. May 23, 1934) is more responsible for pioneering the sound of analogue synthesisers than anyone else. The spooky sci-fi tracks in, you know, every movie ever owe him a particular debt; Moog gear has been used on everything from A Clockwork Orange soundtrack to the soundtrack for Mass Effect 3.
Moog actually got his start building theremin kits in the 1950s, but his company Moog Music is perhaps best known for its analog synths like the Doodle's interactive one. Moog was so smart that even legendary inventor Ray Kurzweil employed him at one point. Moog died in 2005, but his company continues to be one of the most innovative musical instrument makers in the world. In the last decade it has put out the insane Moog Guitar, as well as Animoog, one of the best iOS synthesisers.
The instrument in the Google Doodle is a very simple analogue synth similar to Moog's Minimoog designs. Basically each key is a tone whose wave form is shaped by the various knobs on the panels above. (For more on how analogue synths work, you should see the excellent series of explainers produced by Moog Music, "The Fundamentals of Synthesiser Programming.") Move the knobs around and play different notes and you'll see the Doodle does an admirable job of mimicking the behavior and sound of the real thing. Maybe the coolest feature is that the Doodle allows you to create multi-track recordings of your work on the synth as well as share your creations with a link or—of course—with Google+.