Yoga can be any or all of the following, depending on WHY you want to take it and HOW you incorporate Yoga into your life:
1) a great physical fitness program, involving stretching, strengthening, and elongating the spine for proper alignment of the vertebrae
2) breathing techniques and relaxation, lowering blood pressure, increasing cardio-vascular health, increasing lung capacity, releasing tension and stress, and learning to relax and enjoy life
3) Meditation - to calm the mind, bring emotional balance, mental clarity, focus and concentration
4) the learning of a philosophy, by experiencing emotional tension release from your own body, increasing awareness of what is happening in your own body and mind (many yogis and yoginis notice and feel their heartbeat, circulation flowing through their body, can increase of decrease blood pressure at will, a heightened awareness of what is going on in your body, then you have a heightened awareness of what is going on with your mind). So yoga is EXPERIENTIAL LEARNING - you don't accept dogma as it is told to you - you question everything, until you feel beliefs yourself and understand what is right for you.
5) a philosophy of life - following several basic principles (non-harming, truthfulness, discipline - see my web site under Yoga Information/Yoga Philosophy/Yamas and Niyamas). It is to awaken the "witness consciousness" - the part of you that can step back and observe what your brain is doing, what is happening for you - dispassionately, so you can better understand yourself.
6) A Spiritual Practice - Spiritual fulfillment - as you learn to awaken the witness consciousness, meditate, and increase your awareness, at some point, you realize that you are MORE than this Body, this Mind, this shell - that you are a drop of beautiful energy in a spiritual ocean (to use the cliché, you are one with everything - Spirit/Life Energy/God/Power of the Universe is in you) and this is a truly fulfilling experience when you realize this for yourself. Someone can TELL you about this all they want, but you really need to feel it and experience it for yourself - awakening the divine energy in you.
Take what you need from Yoga. For some people, it is simply a class to take where they get a good stretch and a nice balanced work out. For others, it is a way of life - we talk about being on the Yogic Path, our own hearts leading us where we need to go. Yoga is about Union - the unity of YOURSELF with the LIVING WORLD around you - you are part of the divine dance. It is about releasing tension in the body and the mind, relaxing, and bringing the mind to stillness so you can listen to your heart, so you can learn and grow.
If you are feeling empty right now, you need to think about whether it is a spiritual emptiness you feel. Yoga is NOT a religion, but it does encourage you to connect with your inner spirit and follow its guidance. It is a spiritual practice IF you want it to be. It may help you.