What is YOGA
This is a big subject! There are eight aspects, or limbs, of yoga. Asanas, or postures, cover the physical aspect and they are the most recognisable to us Westerners. Most importantly, the images we see in books and websites are a moment captured by adept yogis. These images, although beautiful, are not the point. Yoga is about moving and finding space in the body. It is about listening to your body and working towards the edge of a movement and not beyond it into pain. End-gaining to reach a picture prefect asana, which can be a pitfall for us all, is not what yoga's about.
Once you begin to listen to how your body moves you step into the realms of mindfulness and meditation. Not only can you realign your body this way, you can begin to release some accumulated stories which are held there. In yoga this is called releasing Kleshas or blockages. As you focus on how your body is moving, the chores and worries of the day step a side and so you find some respite from your monkey chattering mind!
The common cry of 'I'm not flexible enough to do yoga' is an anomaly. Flexibility comes with practice and it's really not necessary as a starting point.