The word yoga means to yoke or unite. Yoga is the practice that unites the body, mind and spirit.
What does this practice entail? Hatha Yoga is the practice of the first 4 limbs of yoga: the ethical rules (yamas and niyamas), the physical postures (asanas), breathing techniques (pranayama). The 5th limb of yoga is, withdrawing the senses (pratyahara). It occurs as a natural outgrowth of the first 4. Raja Yoga is the practice of the final 3 limbs of yoga: concentration (dharana), meditation (dhyana), and bliss-absolute (Samadhi).
Hatha-Raja yoga, the practice of all 8 limbs, is bigger then the sum of its parts.