The sweetly fragrant dried, cured pods of the vanilla orchid (Vanilla planifolia), which is native to Central America and Mexico. (There are other varieties from Tahiti and Hawaii.)
The pods can be used either whole or split to reveal the aromatic seeds, which can be scraped out and added to custards, sauces, and so on. The pods can then be stored in a sugar jar to impart their flavour, or they can be infused directly in custards, creams and milk puddings.
Vanilla extract is an alcoholic solution with the true flavour and aroma of vanilla pods. Those marked 'pure vanilla extract' are the best quality, if more expensive. Vanilla extract can be used as flavouring in place of the real thing, but avoid using products marked 'vanilla flavouring', which lacks the true flavour and aroma of vanilla.