A Voxel is the technical name for the material that makes up Asteroids, Planets and Moons. They are not blocks and are all immobile. They are not affected by Gravity however, they sometimes generate gravity as seen with Planets and Moons. They also are the source of Ore and Ice but must be refined into useful Material before they are useful.
Typically they're mostly composed of Stone but often also have other more precious Ores that can be mined with a Drill. Grinders have no affect on Voxels. Typically Voxels cannot be placed in the same manner as Blocks, but with Voxel hands in Space Master it's possible to add, remove or modify Voxel shapes and sizes.
Most blocks cannot intersect or be placed into Voxels, but Stations can and this is useful to keep them secured in place otherwise they would be free to move (similar to Large Ships). This is useful for stations in space that you do not wish to be accidentally knocked or pulled by docked ships and necessary on planets otherwise the blocks themselves would be affected by gravity and fall.