The main chemical components of saffron are responsible for its taste, aroma and its color. Simplified croc in produces color, picrocrocin produces flavor and adrenal produces aroma.

The color of the saffron is a strong indication of the saffron's flavor. Color is due to the degradation or the breakdown of arytenoids, such as croc in and crocheting.

Coupe or cut saffron is the name given to saffron, which has been cut so that only the reddest tips are used.
This produces a saffron product with a more intense flavour.Whole saffron strands have naturally occurring yellow or whitish bottom ends. These lighter portions do appear in saffron of slightly lower quality.
But where their amounts are kept to a minimum, the saffron is perfectly good. A higher price is paid for saffron that has these lighter ends completely removed, as they add weight, but do not add a significant amount of color or flavor to the dish.

The higher price is justified as less of the coupe or cut saffron would be needed to create the same flavor and color. Sargol is the Iranian name for Coupe.

The saffron crocus is the only crocus which can produce the saffron spice.

Saffron should be soaked for a minimum of 20 min and up to 12 hrs or overnight in a little warm water before cooking. Saffron continues to release its flavor for up to 12 hours after cooking, which is why it is seen as important to soak the saffron strands before cooking.
Soaking or steeping saffron helps to release its full flavor before cooking, in this way less saffron may be needed to achieve the same desired taste.
The highly soluble properties of saffron mean that it can be soaked or steeped in a vide variety of liquids, at a range of temperatures.
It can be steeped in hot or temperate water, broth, milk, white wine, white vinegar, vodka, citrus juice and even flavored waters such as rosewater or orange blossom water.
This is one method, but not the absolute rule - saffron is often dried and then crushed with a mortal and pestle.
And it is often thrown directly into the pot when time is short, whenever there is an extended cooking process or when desired.

Saffron strands will last for years, if stored in an air tight container, away from direct light. 
Saffron powder must be stored in the same way but should be used within one year of purchase.

There is no substitute for the flavor and aroma of saffron.
Saffron substitutes include:
Turmeric, which is a yellow spice, purchased usually in powdered form, though before grinding closely resembles the ginger root. It delivers some of the same color and a little of the flavor of saffron, perhaps at a cheaper cost.

Safflower is a thistle-like flower that has seeds which produce an edible oil and birdseed. The flower was traditionally grown for its red dye .Its appearance and color closely resembles saffron, though it delivers little of the saffron taste.