Garlic powder is dehydrated garlic cloves that have been ground to a white-yellow powder. It provides some of the flavour, but not the texture, of fresh garlic, lasts for a long period of time and disperses well in liquid, hence the usage in marinades. High quality brands consist of pure garlic, while low quality brands may add artificial colour and flavour enhancers.
Garlic powder is often included in dried spice blends, where the most common is garlic salt.
Garlic is believed to be a natural alternative to pesticide and to protect against fleas, and garlic powder is added to pet food with this purpose.
1 clove of fresh garlic can be substituted by 1/5 - 1/8 teaspoon of garlic powder.