A solid timber floor is made from planks of wood that have been fashioned to the right thickness to be used as flooring.
An engineered wooden floor is using timber and other components that are bonded together to make flooring planks that can then be laid.
A floating floor is made using engineered timber flooring boards that are sitting in place. These boards are not nailed in place like traditional solid timber is.
Not really, maybe you can sand it lightly once - but given the top layer is so thin it would not take much to wear through the top layer.