Jnana Yoga is the yoga of knowledge-not knowledge in the intellectual sense-but the knowledge of Brahman and Atman and the realization of their unity. Where the devotee of God follows the promptings of the heart, the jnani uses the powers of the mind to discriminate between the real and the unreal, the permanent and the transitory.
Jnanis, followers of nondualistic or advaita Vedanta, can also be called monists for they affirm the sole reality of Brahman. Of course, all followers of Vedanta are monists: all Vedantins affirm the sole reality of Brahman. The distinction here is in spiritual practice: while all Vedantins are philosophically monistic, in practice those who are devotees of God prefer to think of God as distinct from themselves in order to enjoy the sweetness of a relationship. Jnanis, by contrast, know that all duality is ignorance. There is no need to look outside ourselves for divinity: we ourselves already are divine.
What is it that prevents us from knowing our real nature and the nature of the world around us? The veil of maya. Jnana Yoga is the process of directly rending that veil, tearing it through a two-pronged approach.