Sorry if this is a totally stupid question, but can someone explain the 2D and 3D difference to me?  Once you render a 3D you have a 2D image, right?  So . . . ???  Is 2D something that's created with other than 3D software. 

For that matter, if it started as a photograph but is now a work of art that doesn't resemble photography, does it still go in the photography category?

Thanks!