Green Coffee extract is a supplement and/or food product that is derived from Green Coffee Beans. For all intents and purposes, it has similar chemical composition to coffee beans in general but has a much higher content of molecules known as Chlorogenic Acid; a term used to refer to molecules that have small phenolics bound to a Quinic acid group.
The Chlorogenic Acids in Green Coffee Extract are readily absorbed, and they themselves or their metabolites (such as ferulic acid) mediate many of the benefits of Green Coffee Extract. Supplementing Chlorogenic Acid should also, theoretically, confer much of the same benefits as Green Coffee Extract (and vice versa).
Oral ingestion of Green Coffee Extract may weakly reduce body weight in overweight and obese persons (mechanisms currently unknown, thought to be related to preventing carbohydrate uptake from the intestines after a meal) although the degree of weight reduction seems quite unreliable at this moment in time; studies in lean persons are nonexistent right now. A handful of studies suggest that 'blood health' can be improved via increase vasoreactivity and lowered blood pressure, which have been shown to benefit people with poor vascular function or high blood pressure; this may only be a bandaid effect (with one study noting that 2 weeks after cessation the beneficial changes were being normalized) and may be due to the ferulic acid metabolite.
Green Coffee Extract is indeed healthy, but for the benefits it is touted for it does not appear to be as potent as some other supplements.