The short answer is that all the Yogas follow the same basic principles and postures. Raj Yoga, Ashtanga Yoga and Hatha Yoga basically follow similar techniques. Hatha Yoga focuses more on the strengthening of the body but follows the same path to achieve it. Raj Yoga and Ashtanga Yoga refer to the same thing.
Raj Yoga or Ashtanga Yoga was compiled by Sage Patanjali almost 2300 years ago and is considered by all to be the fountainhead of all other yoga practices. As Yoga evolved, some Yogis focused on some asanas and a particular sequence of asanas and named it after themselves. Others tried to fuse it with other fitness techniques and named it differently.