Aluminum is highly reactive. It is rapidly dissolved by acids (many foods) and alkalis (many cleaning preparations). The impact of aluminum dissolved in food and ingested is the subject of great controversy. A while ago aluminum was a major suspect in Alzheimers disease, but this view is currently out of favor. In any case, aluminum can't be good for the flavor of your food.
Today bare aluminum cookware appears almost entirely in bakeware and restaurant equipment: stock pots, large sauté pans and the like - hopefully used for non-acidic foods.
Aluminum cookware for the home is now pretty much all "hard anodized". This is an acid-electric treatment that builds up a film of extremely hard and non-reactive aluminum oxide, temperature resistant to the melting point of aluminum. This coating may be colored but most anodized cookware is charcoal or black.
This anodized surface is also non-porous, thus quite non-stick, and on basis of the anodizing, this cookware is declared entirely safe, regardless of you opinion of the safety of underlying aluminum itself.
The main problem is this coating, like all coatings, is vulnerable. It can be broken by scratches. While it is very hard itself, it is supported by a rather soft aluminum metal underlay. It is also eventually dissolved in normal use by the acids and alkalis it is exposed to in cooking and cleaning.When you start seeing silvery patches on your cookware, it's time it was replaced.
Anodized Aluminum is used mainly for skillets, sauté pans and other stovetop frying applications. It is used to a much lesser extent for Dutch ovens and saucepans.
Warning: Today all the major manufacturers are selling "infused" hard anodized aluminum cookware and have dropped the plain anodized models. This "infusion" is a polymer, certainly a fluorocarbon polymer, which means these pans have the same heat and toxicity problems of Nonstick Cookware. Since most nonstick cookware is based on anodized aluminum, there is no longer a significant difference between the two.Links