A spice is a dried seed, fruit, root, bark, or vegetative substance used in nutritionally insignificant quantities as a food additive for flavor, color, or as a preservative that kills harmful bacteria or prevents their growth. It may be used to flavour a dish or to hide other flavours. In the kitchen, spices are distinguished from herbs, which are leafy, green plant parts used for flavoring or as garnish.
Many spices are used for other purposes, such as medicine, religious rituals, cosmetics, perfumery, or for eating as vegetables. For example, turmeric is also used as a preservative; liquorice as a medicine; garlic as a vegetable.
A spice may be available in several forms: fresh, whole dried, or pre-ground dried. Generally, spices are dried. A whole dried spice has the longest shelf life, so it can be purchased and stored in larger amounts, making it cheaper on a per-serving basis. Some spices are rarely available either fresh or whole, for example turmeric, and must be purchased in ground form. Small seeds, such as fennel and mustard seeds, are used both whole and in powder form.
The flavor of a spice is derived in part from compounds that oxidize or evaporate when exposed to air. Grinding a spice greatly increases its surface area and so increases the rates of oxidation and evaporation. Thus, flavor is maximized by storing a spice whole and grinding when needed. The shelf life of a whole spice is roughly two years; of a ground spice roughly six months. The “flavor life” of a ground spice can much shorter. Ground spices are better stored away from light.
To grind a whole spice, the classic tool is mortar and pestle. Less labor-intensive tools are more common now: a microplane or fine grater can be used to grind small amounts; a coffee grinder is useful for larger amounts. A frequently used spice such as black pepper may merit storage in its own hand grinder or mill.
Some flavor elements in spices are soluble in water; many are soluble in oil or fat. As a general rule, the flavors from a spice take time to infuse into the food so spices are added early in preparation.
Source and References http://en.wikipedia.org/wiki/Spices