toner is a color used to tone the hair to the shade you want. developer is the chemical used to lift the cuticle to insert the color pigments into the hair shaft.
for example.. lets say you want to go darker.. you'll use a 10 volume developer along with say a level 1N(in otherwords.. black). you use 10 because you don't need a lot of lift to deposit the color pigments into the shaft to darken it.
BUT if you want to go lighter.. say bleach it.. you'll use anywhere from a 20-40 volume developer to lift the cuticle more to REMOVE pigments from the hair.. this lightens the hair.. so if you are going from black.. you are pulling the reds, violets, blues, oranges, etc from your hair to get it to a more yellowed tone (say a 9 N blonde). Now to get it to a more natural, less golden blond you'll want to tone it. So you use the toner to tone it down to a more natural neutral blond.
at 12:07 AM on Mar. 22, 2011