https://www.answers.com/Q/What_is_different_between_secondary_electron_image_and_back_scattered_electron_image

Secondary electron images show morphology and topography of a sample. The more the number of electrons reaching the detector, the brighter the image is. Back scattered electron images show difference in composition - or more exactly, difference in atomic number over a sample. The higher the atomic numbers of the atom, the more backscattered electrons are bounced back, making the image brighter for larger atoms.

A second difference is back scattered electrons are produced by the elastic interaction of the beam electron with nuclei of atoms in the specimen. No energy is lost as it is elastic interaction. On the other hand secondary electrons are produced by inelastic interaction of beam electrons with electrons in the atom rather than the nucleus. Since it is inelastic interaction, there is energy loss.