Frills, ruffles, flounce, froufrou, furbelow... The terminology is sometimes used interchangeably, and to the untrained eye they can look the same, however the pattern for a frill is very different to the pattern for a flounce.
In essence, both frills and flounces are a way of adding additional fullness and volume to a garment, E.g the ruffles on the edge of a flamenco skirt, or the flounce in a peplum.
However, the main difference between a Frill and a flounce is that a frill has the volume added all the way through the pattern piece, from top edge to circumference edge - then it is gathered at the top edge where it is attached to the garment.
In contrast, a flounce - only has additional volume at the lower circumference edge, not all the way through. So the edge that is sewn to the garment is flat, creating fullness but with a smooth look at the seam.
In a regular, mid weight fabric like calico the difference between a frill and a flounce is noticeable, but it isn’t huge. However, as soon as you move to either side of the fabric spectrum (e.g a light weight chiffon or something heavy like leather) the results will be vastly different, and this is where understanding the principles behind cutting the pattern as a frill or a flounce really come into effect.
Fabric choice is generally the most important factor to determine before starting your frill/flounce pattern as it really makes a difference to how the frill/flounce will hold its volume.
Light weight fabrics like chiffon or georgette lend themselves to becoming frills really well, but aren’t very effective when cut as a flounce as the light fabric tends to fall flat. Fabric that is stiff like leather or denim can be tricky to gather at the seam. Therefore the most effective way to create fullness in heavier fabrics would be cutting the pattern as a flounce, these fabrics are also very effective at holding themselves out from the body, giving the illusion that there is much more volume than there really is.
As a general rule of thumb the lighter a fabric is the more volume I add into the circumference.