Unlike curry powder, garam masala does not contain turmeric and it tastes richer and more intense. You can buy it at Indian grocery stores, in well-stocked supermarkets, or online, and it is available as a ground spice mix, mix with whole spices, or as a paste. When it has not been processed, it can be stored longer without any loss of flavor.
From Wikipedia: “Garam masala (Hindi: गरम मसाला, Punjabi: ਗਰਮ ਮਸਾਲਾ,Urdu: گرم مصالحہ, Bengali: গরম মসলা garam ("hot") and masala (a mixture of spices)) is a blend of ground spices common in India, Pakistan, and other South Asian cuisines. It is used alone or with other seasonings. The word garam refers to "heating the body" in the Ayurvedic sense of the word, as these spices are believed to elevate body temperature in Ayurvedic medicine.”
“The composition of garam masala differs regionally, with many recipes across India according to regional and personal taste, and none is considered more authentic than others. The components of the mix are toasted, then ground together.
A typical Indian version of garam masala contains:
Some recipes call for the spices to be blended with herbs, while others call for the spices to be ground with water, vinegar, or other liquids, to make a paste. In some recipes, ingredients including nuts, onions, or garlic may be added. Some recipes also call for small quantities of star anise, asafoetida, chili, stone flower (known as dagadphool), and kababchini (cubeb). The flavours may be carefully blended to achieve a balanced effect, or a single flavour may be emphasized. A masala may be toasted before use to release its flavours and aromas.”
“All of the ingredients are usually first toasted in a dry pan before they are ground so that the essential oils and flavors are released. After cooling, they are crushed with a mortar or ground in a spice mill, but they can also be combined as whole spices. The mix can be stored for several months in a sealed container.
Garam masala has a tobacco-brown color and because of its strong flavor, it is used most commonly to season meat dishes. The mix can either be toasted in hot oil before the meat is added, or added at the end of cooking (as is usually the case). Many Indian recipes use garam masala sparingly and as an ingredient that helps to bring out the leading flavor in the dish. For this reason, most recipes call for additional spices.*”
Preparing garam masala yourself:
You can easily make garam masala on your own by using the main ingredients listed above. Here is one recipe you can use:
1 tbsp cumin seeds, ½–1 tsp coriander seed, 1 tsp black pepper (freshly ground), 1½ tsp ground cardamom, 1 tsp cinnamon (you can also grind a portion of a cinnamon stick), 5–6 cloves, and ½ tsp ground nutmeg.
Mix the ground ingredients and then store in a suitable container. This recipe makes approx. 100 mL (12–15 servings) and will keep for several months if stored properly (in a cool, dark place, protected against moisture).
You may toast the ingredients for 15 minutes on low heat before grinding to increase the flavor, but the shelf life will then be reduced.
Note (italics): * = Translation from a German Wikipedia entry