Truffle oil is a light cooking oil, such as olive or grapeseed, infused with truffles so it bears the truffles' aroma and flavor. Many inexpensive truffle oils do not use real truffles, but instead relying on a chemical derived from truffles.
Truffles are flavorful fungi similar to mushrooms that grow underground near certain species of trees. Truffles are rarer and more difficult to find than mushrooms. Considered a delicacy, truffles sell for as much as $30 to $75 per ounce. A cook often sprinkles truffle oil over a finished dish just before serving so that the aroma is at its highest.