Castor oil is a vegetable oil that is obtained when the seeds of the castor oil plant are pressed. In this case, instead of naming the oil after the plant it is extracted from, the plant is actually named after the oil! It is typically a yellowish color, and tends to be pretty thick when poured, unlike many of the other oils we use for soap making.
Castor oil can be ingested, and has been used traditionally as a laxative, among other medicinal uses. Clearly, these properties of this oil are not what we will be attempting to include in our soap! In lip butters or other topical applications, it acts as a humectant, meaning that it helps to bring moisture to the skin. In soap, castor oil is a great bubble booster, and can be used in high percentages in shampoo bars. In regular soaps, we typically do not want to use such a high percentage, because castor oil can also speed up trace.