Ceramics are made from something called slip. Slip is a mixture of clay and water mixed to a creamy consistency. There may be a few other inorganic substances mixed in to give the final piece strength, or to keep it from clumping up for ease of working.
Slip is mixed up and then poured into a plaster mold. The water is absorbed by the plaster and the slip begins to harden around the mold. When it is determined that the clay body is thick enough, the excess slip is poured out and the mold is set aside to let the remaining slip dry. This process is called slip casting.
When it is dry enough, the mold is opened up, revealing the clay body.