We are all aware that clay is involved in various art forms but do you know that all clay is not the same. Every piece of art has its own requirement and hence different types of clay.
Water based clay is natural clay mixed with water. It is an inexpensive type and must be covered or it will dry out. With the right combination with water it is easily malleable and convenient to remove from a mold. However if it is a long project this clay may not be the wise choice as it can sag and easily separate so will be very difficult to use as a combination with plastics or polymers. This is used in ceramic sculptures after firing.
Oil based clay is not soluble in water as it is infused with wax and oil. It is sometimes made by the artists themselves to cut down on project cost. It does not dry and can be remolded. Some who do not like the rough dry feeling of water based clay on their hands love this type as it feels soft and does not suck moisture from their hands. The biggest disadvantage is that unlike the first type you cannot make permanent work with this type as it is not fired. It does have an advantage if you want to take your own time while working because it won't dry up.
Polymer clay is not actually clay but a synthetic material which is malleable like clay and can be fired in an oven at home. This is very popular while making home craft or students projects or just as a constructive family activity.
Kid's clay is known commonly as "play doh". These are tested first in order to not have any ingredients that can harm children. This is also quite inexpensive and can be easily manipulated by the kids.