I read that there are something called black garlic out there. What is the common usage of black garlic? What is the difference of its from common garlic?
Black garlic is slow-cooked and fermented using a process developed in Korea. In Southeast Asia, it is sold as nutritional supplement (in energy drinks, etc.) more often than as an ingredient. The garlic is charcoal black in color, and tastes mostly of the caramelized sugars in the original garlic, and only faintly garlicky. Think of it as a sort of sweet garlic jam.
Since black garlic is essentially a sweeter, less garlicky roasted garlic, it could be used in place of roasted garlic. This would include salad dressings, pasta, and pizza. Obviously, it can also be used for Korean, Thai and Vietnamese food. While I haven't had it this way, I imagine it would give a terrific flavor boost to Vietnamese or Korean clay pot dishes.
That being said, I don't buy black garlic despite it being fairly easy for me to obtain (I live in San Francisco). The flavor isn't that much better than regular roasted garlic, it's far more expensive, and it looks unattractive.
EDIT: Black garlic starts out as regular garlic and is put through a fermentation process that besides changing the color of the cloves to black, also mellows the flavor and reduces the sharpness associated with raw garlic. The texture of the cloves become very soft, also. While I have never used nor eaten black garlic, a website, linked below, is dedicated to everything related to black garlic, including how to use it in recipes.
Blackgarlic.com has a wealth of information on how this fermented version of garlic is made, its unique qualities including taste, texture and chemical properties, where to get it and...recipes!
NEW EDIT: And to balance out the linked info. so there's a non-commercial reference, I found this very in-depth article on black garlic's attributes that includes a reference to a scientific study: http://www.wellbeing.com.au/newsdetail/Black-Magic-Garlic_000553