There really isn't just one answer. Different programs have different guidelines, and not everyone agrees on what low carb means. A good start is The Low-Carb Bible, by Elizabeth Ward.
It describes alot of lower carb programs including Atkins, Carbohydrate Addicts, Neanderthin, Protein Power, Schwarzbein Principle, South Beach, Sugar Busters, Suzanne Somers, and The Zone
People following Neanderthin, The Zone or South Beach plans, generally do not consider the programs "low" carb, but "right carb," but they're still included in the book because they're generally lower in carb than the traditional American diet.
Atkins goes as low as 20 - 60 g carbs. Personally, I aim for about 100 g and eat South Beach/WW Core-style.