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. “Mudstone” should not be used to refer to a variety of fine-grained, siliciclastic sedimentary rock (that term is often used to refer to a fine-grained variety of limestone).
Shales vary in color from black to gray to reddish to brownish to greenish. Purplish & bluish colored shales are also known.
Shales are composed of clay minerals. The clay minerals are a large group of (mostly) complex silicates, the result of weathering of other silicate minerals. So, shales are composed of clay-sized grains of clay minerals.