Sweet almond oil is a fixed oil while bitter almond oil is considered an essential oil.   Fixed oils are the carrier oils we use as a base oil whereas essential oils are extracts from plant material. Fixed oils don't evaporate and are typically what you think of when you hear the word oil.  Sweet almond oil is actually a vegetable oil. Essential oils can evaporate and are not really oil at all as they are the \\"essence\\" of the plant material from which they come.  They are generally used for scent, flavor, and aromatherapy properties.