Hair extensions come in 2 different versions, natural (made from real human hair) and synthetic (manufactured from synthetic fibers). Each come with their own relative pros and cons.
Natural human hair comes from a donor, someone that has "cut off" their own hair. Typically it is sold to a reseller of hair extensions, one who may offer both natural and synthetic hair extensions for sale. Natural hair extensions have the obvious advantage that they are actual human hair, they look and move just like natural hair would, they can be styled using heat or chemicals and react as normal does. A disadvantage to natural hair extensions is cost; It is much more expensive to procure real human hair for use in extensions than it is to manufacture a synthetic product in a lab. It may also be difficult to correctly color match natural hair to the existing hair of the recipient, since even slight variations in color can be quite noticeable when hair is lengthened with extensions. For more information about natural types of hair extensions, visit the artificial hair integration wiki