Sedimentary rocks form by the solidification of loose sediments. Loose sediments become hard rocks by the processes of deposition, burial, compaction, dewatering, and cementation.
There are three categories of sedimentary rocks:
1) Siliciclastic sedimentary rocks form by the solidification of sediments produced by weathering & erosion of any previously existing rocks.
2) Biogenic sedimentary rocks form by the solidification of sediments that were once-living organisms (plants, animals, micro-organisms).
3) Chemical sedimentary rocks form by the solidification of sediments formed by inorganic chemical reactions. Most sedimentary rocks have a clastic texture, but some are crystalline.
Shale is the # 1 most common sedimentary rock on Earth. Shale forms by the solidification of very fine-grained sediments (mud & clay). Clay-sized grains are all sediments smaller than 1/256 of a millimeter in size. Shale feels relatively smooth to the touch (smooth for a rock). Almost all shales weather & break apart as relatively thin, flat pieces, a physical property referred to as “fissility”. Some very thinly-laminated shales weather into almost paper-thin pieces - they are called paper shales.
“Shale” is supposed to be an informal field term only. Technical rock names for shales include mudshale, clayshale, claystone, mudstone, and mudrock. Each of these terms has a different meaning. Of these terms, "mudstone” should not be used to refer to a variety of fine-grained, siliciclastic sedimentary rock - that term usually refers to a fine-grained variety of limestone (I would recommend using two word terms to prevent confusion - "siliciclastic mudstone" for the non-fissile shale and "lime mudstone" for the fine-grained limestone).
Shales vary in color from black to gray to reddish to brownish to greenish-gray. Purplish- and bluish-colored shales are also known.
Shales are composed of clay minerals. The clay minerals are a group of silicate minerals that form by chemical weathering of other silicates. So, shales are composed of clay-sized grains of clay minerals.