They are called “Doodles”. They first started in 1998 when Google founders Larry and Sergey played with the corporate logo to show their attendance to the “Burning Man” festival in the Nevada desert.
A stick figure drawing was placed behind the 2nd "o" in the word. The positive response was so big that they assigned a whole team in the year 2000 to develop new doodles for many celebrations.
Now you know how to enjoy them, just click on the “I’m feeling lucky” button. Give it a try!