Backscattered electrons (BSE) are primary beam electrons that, after interacting with the sample, follow a path back out of the sample.  They are generally considered to be emitted from the upper 1/3 of the beam analytical volume and their numbers (ie., relative brightness on the CRT screen) depend on the average atomic number of the material.  Thus, this secondary signal is the best method to image multicomponent materials.  See also this link.

See a comparison of BSE and SEI images, here.

Ol = olivine, avg Z = 11.6

Pl = plagioclase, avg Z = 10.6

Cpx = clinopyroxene, avg Z = 11.1

Sp = spinel, avg Z = 11.2

Ol

Pl

Cpx

Sp

250 um