Hair extensions attach to your natural hair and add length and volume. Whether you have shorter hair and want to add some length or have thin, lifeless hair and want to add body, hair extensions can help you achieve your desired style. According to the Hair Loss Expert website, there are two types of 40-inch human hair extensions: synthetic and human hair. The human hair extensions are of higher quality and ensure the most natural look. You can style them the way you would your natural hair, including curling.