Carrier Oils are basically Vegetable Oils derived from the fatty portion of a plant usually from the seeds, kernels or the nuts. They are mainly used to dilute essential oils and other oils prior to application. They carry the essential oil onto the skin as direct application of essential oils, absolutes and other concentrated aromatics can cause severe irritation or reactions in some individuals.
Some carrier oils, such as olive oil, may have a slight aroma, but they generally do not have a concentrated smell and also do not evaporate. In fact, a carrier oil does not evaporate the way that some essential oils evaporate, so it can actually improve the absorption of the oil. They mostly have a faintly sweet, nutty aroma. However if a carrier oils has a strong, bitter aroma it is an indication that the oil may have gone rancid over time.
Majority of the carrier oils are extracted using a "Cold Pressed Method" in which no external heat is applied in order to remove the oil from its oil containing cells and this fact should be taken into consideration while purchasing carrier oils.