Those who participate in organized religion accept their denomination’s deity or deities and worship through a system of long-established rituals. They may read sacred texts that outline a moral code, which they follow, and they may attend worship meetings lead by religious leaders who have been ordained by an authority in that religion.
By contrast, we can define spirituality as the quest for understanding of ourselves and our place in the universe. Many use organized religion as the conduit for their spirituality, but spirituality can also exist outside the bounds of religion. In other words, spiritual practice is essential to religion, but religion is not essential to spiritual practice.
Yoga does share some things in common with religion, including the study of ancient texts and gathering of like-minded individuals for study under a learned teacher, but these things alone do not constitute a religion. Though some yoga practices encourage meditation on a universal spirit, which may even be called God, the nature of that God is left open to interpretation. Some are confused by yoga’s relationship with Hinduism, since yoga’s language borrows from the Hindu lexicon. Though yoga and Hinduism are both products of ancient India, they have evolved into separate practices as yoga (particularly hatha yoga, the study of yoga postures) has spread outside of its land of origin.
Yoga can be a spiritual practice, but it is not a religion, because it does not dictate the nature of a God to be worshipped.