Ruche/Ruching (roosh/ing) derives from the French word for "beehive" and refers to a strip of pleated or gathered fabric used to finish an edge or for trimming (ruching is a noun rather than a verb). Kind of sounds like a ruffle, doesn't it? Wondering what the difference might be between ruching and ruffles, I did some sleuthing on the internet to see what I could learn. I found many, many examples of ruching used on historical garments (incidentally, much of what is called ruching these days is actually shirring or gathering) and the key feature of ruching seems to be that it is fairly narrow and applied on top of fabric as a separate element rather than inserted as part of the construction. As such, ruching does seem to fall under the category of a ruffle, a very specific type of ruffle and application.
I also found a paper ad from 1870 advertising ruchings as an item for sale along with fringes and dress trimmings. To me, this sounds like it may have been a product like gathered or pleated ribbon.
Below are a couple of images showing how ruching can be used.
Ruffles are strips of fabric gathered along one side (some variations are gathered in the middle of the strip) and attached to a fabric surface or in a construction seam. Ruffles can be purely decorative or a material part of the sewn project.
Shirring consists of multiple parallel rows of gathering stitches that are pulled up to gather the fabric and give it shape. A popular application of shirring is to use elastic in the bobbin so that when fabric is sewn, it gathers and creates a dense stretchy material that is then used to make sundresses and skirts.
Smocking is embroidery stitched on top of pleated fabric. The embroidery stitches are applied along the folded edges of the pleats to hold them in place, typically creating a decorative design in the process. For more information on smocking, please read Introduction to Smocking and Smocking Techniques.
Tucks are pleats that are sewn in place. Pintucks are small scale tucks that are often arranged in multiple rows. The tuck technique is one that is frequently used in heirloom sewing. From a utilitarian point of view, tucks are useful for shortening garments such as skirts because they can be let out as needed.
This image shows how pintucks are sewn in. To make sewing pintucks easier, you can use a pintuck foot that has grooves to line up the stitches so they are straight.
This photo shows a neat technique with pintucks where stitches are taken across the pintucks in opposite directions to pull the flap either up or down. I love the look of this.
I hope you will be inspired to try one (or more) of the fabric manipulation techniques and broaden your sewing horizons!