Shale is similar to siltstone but with even finer grain size, less than 1/256 of a millimeter in diameter.
It is a very common sedimentary rock.
It is composed of silt or clay. This is very fine particles of weathered and deconposed rock.
Once deposited the silt becomes compacted and cemented together into solid stone.
Silica, calcite, and iron oxides are the most common cementing minerals for shale. These minerals are deposited in the spaces between the silt grains by water.
Over the course of thousands or even millions of years the minerals fill up all of the spaces. Shale is usually found in layers.