On July 7, 1947, some sort of object crashed on a ranch near Roswell, N.M. The next day, July 8, Roswell Army Air Field (RAAF) public information officer Walter Haut issued a press release stating that "personnel from the field's 509th Operations Group had recovered a flying disk" from that crash.
Later that day, however, the press reported that Commanding General of the Eighth Air Force Roger Ramey had stated that it was, in fact, a weather balloon that was recovered by RAAF personnel. A press conference featuring debris said to be from the object seemed to confirm its description as a weather balloon.
Since then, though, the incident has become the stuff of legends and conspiracy theories over a cover-up of an actual UFO landing. Roswell has been memorialized, so to speak, in TV, movies, and books.
The Doodle is more than an animated .gif or short video. It is actually a game of sorts. When a visitor (not alien, we assume) goes to the Google homepage, they see a UFO that seems to be smoking and descending. Click the play button and the game begins.
After crashing, you will notice a UFO icon in the upper right hand corner. If you find the correct three objects, you will be able to fill that up and win the game. For example, when the alien first appears, you will head down a hill and pick up the first part of your ship.
In addition to objects you will fill up the UFO icon with, you will also find and pick up objects that -- instead of being a part of the spaceship -- will be used in part of the adventure.
If you're stuck, here's a direct path to how to win.
After you've gone down the hill and picked up the first piece of your ship, go to the left. You'll find a cow. Click on its tether and the rope will enter your inventory. The cow will then will walk over to a round patch on the ground, and appear to begin eating, which open an entrance to a cave. Enter the cave.
Inside the cave, you'll find a can of radioactive fuel. Use it on the plant in the cave, which will grow a la "Jack and the Beanstalk." You can then climb out of the cave. Observe that the fuel is still in your inventory.
Go right. You run into a barn. Climb up the ladder and grab the horseshoe. It will automatically be tied to the rope you already have. Move to the right, and you'll see a piece of your ship above the barn window (which, if you open it, houses a horse). Use the rope and horseshoe on the ship to collect the second portion.
Click on the sack of chicken feed on the ground next to the window.
Head right and you will reach a house. Keep going right until you reach a chicken. Use the chicken feed on the chicken and it will leave a feather, which you should pick up.
Use the radioactive fuel cannister on the tree, which will grow like the plant. Climb the tree.
You'll see the farmer sleeping (on the roof?). Tickle him with the feather, and he will drop third and final part of your flying saucer.
Click on it, and you'll then zoom off into outer space.
As you do, a spinning newspaper will appear with news about the flying saucer.
Click on that, and you'll receive an actual Google search about the Roswell incident (which will likely bring up stories about the Doodle).
This is just the latest Google Doodle, which the company frequently "decorates" its homepage with. The company even dedicates a site page to past Doodles.