What is the main difference between mineral oil and castor oil?

There main difference would be their source.

Castor oil is a fixed oil expressed from the seeds of Ricinus communis (family Euphorbiaceae); a purgative.

Mineral oil is a mixture of liquid hydrocarbons obtained from petroleum, used as a vehicle in pharmaceutical preparations; occasionally used as an intestinal lubricant; can interfere with absorption of fat-soluble vitamins.

Synonym(s): heavy liquid petrolatum, liquid paraffin, liquid petroleum