The word 'Yoga' is derived from Sanskrit root 'yuj samadhou' meaning total concentration of mind. Mind is concentrated upon itself and upon nothing else. It's meaning becomes more clear when we read the 2n aphorism of Patanjali's Yogasutra, which teaches that Yoga is nothing but controlling or eliminating all thoughts from mind while practicing Yoga. The 3rd aphorism tells that when mind is devoid of all thoughts, the seer or practitioner of Yoga, rests on his true self or soul.
 By self, we mean our gross mind and body, name and fame. Simply we mean our personality. But this is our external or worldly self. In spiritualism, self is devoid of gross mind and body, name and fame, society and world. Self is the subtle boy and mind. It is the real identity and real existence of a being. Body and world is destructible, but self is indestructible and eternal. It transmigrates from one birth to other. Self is the only divine power in the world.
 Yoga includes both philosophy and some postures to practice. Without philosophy practicing of postures is meaningless. Postures are practiced to keep body healthy and disease free and to keep mind clear and transparent by eliminating all ill thoughts. After long years of practice, a seer realizes his true self. True self is the storehouse of enormous energy and never ending bliss. One also realizes divinity that is the locus of all beings and world.