
It is just re designing of Google logo on some specific occasion. Doodles history in details can be found at the official website. It was started when Larry Page and Sergey went to burning man festival in Nevada, USA. They redesigned a logo [change in second "O"] to make people aware that the founders are out of office. As it was later became a regular event and now a team of Googler Doodler gather from time to time and create new logos for the company for USA or international events. Till now more than 400 USA and more than 700 international doodles have been used in the website interface on various occassion.
Now the company also organize regional and global contest for Doodles, inviting entries from the school students of different class levels.
To know more about the event, participating, proposals, prizes and judging etc one should visit the official page of Google 4 Doodle.