What is a basic difference in between base oils and essential oils?

A base or carrier oil is an inert natural oil used to dilute essential oils (many of which should not be used full strength on the skin) for massage or other delivery.

An essential oil is an oil extracted from an herbal source (leaf, stem, flower, bark, root) by one of several methods. Steam distillation and expression are the most commonly used. The essential oils contain the constituents and chemical components.