I would agree with David's description with the addition that there are several bleaching formulas that are available for use. One that I know of is an Iodine formula and it is used by some printers with excellent results. The only difficulty that I have heard is that since it is iodine it is more difficult to determine the degree of bleaching obtained.
Of course one can buy premixed Farmers Reducer and it seems to work. Additionaly I have come across this formula from Lynn Radeka and I will post it below since it gives somewhat greater flexibility then Farmers in that it is faster acting if needed, but also can be diluted if more time is desired for control of it's action.
My initial reaction was that it was somehow less "pure" then the purely technical way of proper exposure, correct burning and dodging etc. I have seen the results that some really fine printers obtain and there is no denying the possibilities that it affords. However, the effects are not reversible. Now for the other recipe.
Solution A 500 gm Sodium Thiosulfate to 1 litre of water
Solution B