What does natural scene image data-set mean? I came across a paper that is trying to classify and detect object(s) present in what they call a natural scene data-set. They have images containing objects like cars, bikes and people.

Can anyone tell me how this is different from other data-sets that contain the images of cars/bikes/people?

The site with the data-sets I am interested in is: Natural Scene Database.
For comparison, the Caltech Computational Vision data-set also contains cars and bikes from the rear and side view (with less background).

I want to know what is so special about the natural image data-set compared to the standard data-sets containing various viewpoints of cars/bikes from these set of images? Why would this natural image data-set be a bigger callenge in object detection?
From one perspective, not much. However, biological perceptual systems have evolved in response to physical properties of natural environments. And such environments possess certain characteristics, such as Laplacian distribution shaped histograms. Therefore natural images are grouped and analyzed together.