I am not flexible - can I do yoga?

Yes, you are a perfect candidate! Many think they need to be flexible to begin yoga but that's a little bit like thinking you need to be able to speak spanish in order to take spanish lessons. Come as you are - you will find that yoga practice will help you become more flexible.

Is yoga a religious practice?

Yoga is not a religion - it is a philosophy that began in India more than 5000 years ago. Yoga scriptures provide a framework for mastery over the physical and mental body. Due to its roots (geographically, timely) yoga sometimes interweaves other philosophies such as Hinduism or Buddhism but you don't have to study these in order to practice yoga. Nor is it required to surrender your own religious beliefs to practice yoga.

How is yoga different from stretching or other kinds of fitness?

Unlike stretching or fitness, yoga is more than just an array of physical postures. By connecting the body movements to the rhythm of our breath, the fluctuations of the mind are reduced and we are able to direct our attention inward. Through this process of inward attention, we learn to become more aware of our experiences from moment to moment. This awareness is what makes yoga a practice rather than a task or a goal to be completed. Your body will most likely become much more flexible and simultaneously strong/steady by practicing yoga and so will your mind.