The supreme calling of every human being is to aspire to self realization. All other obligations are secondary. ~Anandamayi Ma
Yoga: the realization (in direct experience) of the preexisting union between the individual consciousness and the universal consciousness.
The four traditional schools are: Jnana Yoga, Bhakti Yoga, Karma Yoga, and Raja Yoga. Focus on a single path is uncommon, although different paths can suit different temperaments and different stages of the soul’s evolution. The four paths are all a part of the whole which is called Yoga. Virtually all people have a predisposition towards one or the other, and will naturally want to emphasize those practices.
Jnana Yoga is the path of knowledge and wisdom, of introspection and contemplation. It involves deep exploration of the nature of our being by systematically exploring and setting aside false identities. The goal is absolute Truth, non-duality. Self-enquiry is the highest and most direct tool. Jnana Yoga generally appeals to the philosophical and intellectual temperament.
Bhakti Yoga is the path of devotion, love, compassion, and selfless service to the Divine, and to all others as the Divine. All actions are done in the context of remembering the Divine. The goal is Prem—pure, conditionless, divine love, and total surrender to the Divine. Bhakti Yoga generally appeals to the emotional temperament.
Karma Yoga is the path of action, selfless service to others, mindfulness, and remembering our innermost being while fulfilling our actions or karma in the world. The goal is sama drishti, complete selflessness, and the elimination of the triad of doer, doing and deed. Karma Yoga generally appeals to the active temperament.
Raja Yoga is the path of self control and self mastery. It directly deals with encountering and transcending thoughts of the mind. The main practice in Raja Yoga is meditation. The goal is to control the mind until it becomes perfectly still at which time there is no longer a wall between the meditator and his or her own Divine nature. Raja Yoga generally appeals to the mystical and scientific temperaments.
Kundalini Yoga aims at purifying the physical and subtle-body systems, and then awakening the Divine power (Kundalini Shakti) residing in the muladhara chakra at the base of the spine. Knowledge of Kundalini yoga is extremely useful in all other paths of yoga, because every person will go through a Kundalini awakening on their way to self-realization, and Kundalini arousal can occur in any path. Kundalini Vidya is the science of human transformation, and as a path unto itself, uses Raja yoga and Hatha yoga practices.
The "Modern" Yoga Class can cause some confusion about the four paths of Yoga. It often focuses mostly (if not completely) on aligning breath with physical asanas. By referring to asana classes as yoga classes one is left with the false impression that this, unto itself, is the meaning of Yoga. Asanas are a small, though surely useful, part of Yoga. Hatha yoga purifies the mind and bodies through the breath and asanas. It also aims at opening and detoxifying the systems through a variety of Ayervedic practices, and often goes hand in hand with Raja yoga and Kundalini Yoga.