Garlic powder is ground, dehydrated garlic. It is a very common seasoning. Applications include pasta, pizza, ranch dressing and grilled chicken.
Garlic salt is simply salt plus garlic powder. (Pre-made products usually include an anti-caking agent.)
Garlic powder is a common component of spice mix. It is also a common component of seasoned salt. Garlic cloves are peeled and sliced. In most cases, the garlic is then heated to a temperature of between 150° and 160 °C (~300-320 °F). The water is removed to a moisture content of about 6.5%. The dehydrated garlic is then further sliced, chopped, or minced until the powder is reduced to the desired particle size.