A shape generator defines the shape of the terrain. Pretty much it's just "that is air and this is a block". Later a composition generator fills the places with blocks in with whatever he think should be placed there. I guess nowadays the height generator pretty much is a shape generator
if you first set the shape generator to "Heightmap". A heightgenerator previously told the generator what the highest block was if I'm not mistaken.
You can find the options for the biome generator
here. I know it's not pretty to redirect you to the source, but sadly that's pretty much the best we have. Here are some examples from the options:
Voronoi,
distorted voronoi,
grown. Constant should speak for itself, it generates a single biome, checkerboard generates biomes in a checker board format. MultiStepMap is pre-1.7 biomes. Twolevel is pretty much 1.7 biomes but without rivers. Grown and GrownProt are 1.7 biomes, but GrownProt a little bit slower, but is simpler to understand as a programmer.
We have two redstone simulators. Incremental, which is the default and best simulator we have, and noop which does.. well.. nothing
On a normal world the default settings would do nicely. As far as I know it's the best settings you can have