The key difference is that raja yoga concentrate on "asanas" or poses to prepare the body for extended meditation.
On the other hand, Hatha yoga creates harmony in the mind and body through exercises; breathe control as well as relaxing the mind by relaxing and meditating. Adopting different poses can ease or prevent health troubles from constipation to cancer.
Raja yoga is known as king of yoga and is also referring to as "Classical yoga" or "Astanga yoga". It is a practice that has to do with the stimulus and harmony of the mind from yoga poses. Raja yoga is also able to manage your feelings in addition to getting your bodies into raja yoga postures. In Indian civilization, the mind lords over the body hence raja.
Astanga, known as eight limbed yoga and is the eight objectives of raja yoga to become a raja yoga master: "Yama" is the application and observance of being moral. "Niyama" is staying true to your own spirituality. The practice in India is believed to be abiding by religious traditions and festivals.
"Asana" is subjecting your body to the various poses to achieve relaxation and increase flexibility. "Pranayama" is keeping to a correct breathing technique by not constantly thinking about the past, as it results in superficial breath. "Prana", generally refers to "life force" in Hindu. "Pratayahara" is through forming your own strength of will and mind without subjecting to outside influence.
"Dharana" is observing of meditation, "Dhyana" is to maintain self detachment. This is by far the most difficult objective of raja yoga to clarify. An absolute ideal is to be free of any emotional or materials attachment to be one and harmonize with the God and the Universe even depriving yourself of food, oxygen and sleep. Once your body attains flexibility, it will align with your needs and desires. "Samadhi", this is the highest state, to be one with God, the Nirvana state of raja yoga.