In the story, the Maltese Falcon is a priceless gold statue encrusted with jewels (a gift from the Island of Malta to the King of Spain), which was lost at sea and later rediscovered, only to be covered in a black paint which made it look more like a common statue.
In the movie, the Falcon is never actually shown on camera until the last few minutes, although it is seen wrapped in newspapers for a portion of the film. However, the way it affects people, bringing out their greed to the point of murder, is what moves the story.