The 'clay' used by a potter is actually a mixture of various clay minerals plus other materials. The other materials are present in the clay naturally or are added to improve the workability  of the clay and the and fired properties of the product.
CLAY MINERALS such as kaolinite, illite and chlorite all have water bonded to the molecular surfaces. High heat can drive this water off to create new molecular structure which does not admit water.
Clay minerals have leaf or blade-like forms which gives them a large surface area. This large surface area allows for many molecular bonds to hold water and other molecules onto the clay.
TEMPER is any material added to stiffen the clay as it is worked. If a clay is too soft it will flop easily. Tempered clays can stand higher before collapsing and be thinner before buckling uder their own weight.
In addition, clay without temper dries very slowly and may crack when it does. Temper helps the clay body to get rid of water and resist shrinkage and cracking.
Temper is typically sand, crushed rock  or grog - which is crushed sherds of fired pot.
Picture
ROMAN-BRITISH BLACK BURNISHED WARE There is very much sand temper in this pot fabric. Smallest scale interval is millimetres. ( From Worcestershire ceramics online database.)
Examination of the make-up of a pot is known as FABRIC ANALYSIS.