A poplar wand is any wand that is crafted out of wood from a poplar tree. More specifically, a poplar wand is any wand that has a poplar shaft, a poplar handle, or both.
Poplar Trees
The defining characteristic of the poplar trees use in crafting wands is that their leaves tend to flutter in the slightest breeze. For example, "quaking" of the poplar leaves is how the Quaking Aspen gets its name. This is also why poplars have the elemental Air.