It is hard to explain the concept of yoga
briefly, as it is part of a long spiritual Indian heritage that has
spanned many writers, and traditions, and has gone through a lot of
changes over time.
Yoga as it is practiced in the West is often times simply the asanas, or yoga postures. And there is nothing wrong with this.
This site presents a lot of information on yoga postures, including diagrams, on many postures. And information on the importance of the breath, which is integral to yoga and which differentiates it from other forms of exercise.
There are a lot of benefits to
doing yoga postures. You certainly don't need to take yoga any further
than that, but there is more information on these spiritual and healing
aspects if the interest is there. Like anything, take what is
appropriate for you, and leave the rest.