There are three main chiseling types: Slope, Bevel and Square. Once you have chiseled a block to a certain type, you cannot chisel that block with any other type.
Every block are subdivided into 8 smaller blocks which you can poke with your chisels. It’s basically one sub-block for each corner of the block. If you use a basic chisel, or use a precision chisel without the precision chisel epic, you will hit 2 of these corners/sub-blocks at once.
The basic chisels are Stone(Square), Copper(Slope), Iron(Bevel).
The precision chisels are Silver(Slope), Gold(Bevel) and Titanium(Square).
The following link has an image that give you an idea of how you can shape a block with a chisel.