A facet is a side – the side of a stone for example, or an element of consideration.
In stone, and any hard material a facet is a cut side, one that has been made with the sole purpose of reflecting light. A multi-faceted stone, such as the Swarovski crystals we use for beading, are sparkly because they reflect the light from many facets.
Bugle beads can be smooth or faceted, as can sequins and some seed beads; glass, crystals and precious stones are either tumbled and irregular or evenly cut then faceted.
When you want to create a point of brightness, to gain the maximum sparkle from a modicum of light, always select beads with facets.