Most modern practitioners initially come to yoga by doing yoga postures or yoga asana. But yoga asana is not yoga. An asana is simply a physical shape that you do with your body and translates to "seat" in Sanskrit.
We use yoga poses to do yoga. But the yoga poses themselves are not yoga.
Think of yoga as a destination which you can reach by car, walking, train or plane. How you get there is not the destination but only a vehicle used to reach your destination.
Likewise, asana is a way to fun and physical way to practice yoga, but it is not the yoga itself. And there are other methods of exploring yoga, such as pranayama, breathing exercises. In other words, you can do yoga without doing asana.