Two poses that often get confused or interchanged are vajrasana (thunderbolt pose) and virasana (hero pose).
Both poses are seated postures, often used during meditation before or after a yoga asana practice. These postures are also good to use during pranayama practice.
The picture on the left is vajrasana and the picture on the right is virasana.
As you can see, the two poses look very alike. Generally, your body is in a similar position. The main difference, however, is where the feet are. In vajrasana, you are sitting on the heels of your feet. In virasana, you are sitting between your feet. The subtle difference changes the elevation of your hips.
The benefits of the two poses are pretty much the same, the elevation of the hips however may cause more or less comfort. Similarly, sitting on the heels or between the heels may depend on the comfort level of the yogi.
Ultimately, whichever posture you chose, you will benefit. Whether you use this posture for meditation, pranayama, or simply as a break between yoga asana, you will find it to be a great posture for your feet and thighs. It also helps create awareness of posture.