Question:
What in the world are these?
Okay, I know they are fabrics, but what kind?
I keep seeing people referring to them in forums and selling sites, but even though I've looked closely at photos I'm still not sure what they are...
I'm guessing knits, maybe polyester? But what's unique about them? How do they compare to 'known' names?

Elaine
Answer:
They sound like ingredients that I might find in my kitchen, don't they?
I found a site that has most of the fabrics for sale and also has descriptions. Fortunate for me because I had no idea that buttermilk was a fabric and I thought onionskin was paper.
Descriptions from http://www.emmaonesock.com/
Buttermilk (poly/lycra) - This is a good fabric for beginning sewers. Buttermilk knit is a medium weight knit with moderate stretch in 2 directions. The printed design only appears on the right side of the fabric; the back of the fabric is generally a solid color. This fabric, when stretched, recovers with a “snap”, making it perfect for travel or for garments you do not wish to iron.
Single Span (poly/lycra) – Single Span is very similar to buttermilk but a bit lighter and silkier; a drapey, opaque, soft and silky lighter weight knit with 4-way stretch and great recovery.
Mesh knits (nylon) - Mesh knits are among the most comfortable fabrics on the market. The sheer quality of the fabric provides a coolness in temperature when the garment is worn. Mesh often is printed with trendy or classic colors and prints, and has a variety of uses. This fabric is a summertime staple for tops, and a cool weather staple for trims and embellishment. It is easy care, fun to wear, and easy to sew. (Pressing seams can be difficult as the fabric tends to curl when cut.)
Onionskin (polyester) - Onionskin knit is another “beginner’s knit”. The fabric is generally printed on the right side, with a subtle pattern on the backing that resembles the appearance of an onion skin. It is very easy to sew! Onionskin is a semi-sheer fabric with a crosswise stretch. The prints available are sometimes funky and trendy, sometimes romantic and soft. This fabric is easy to sew and easy-care.
Answer:
I wonder if these are just names Linda/EmmaOneSock made up for her site? Or if other sites use them?
I've seen them referred to at PatternReview.com and at a yahoo fabric coop, but maybe they are just using the emmaonesock names.
Most of it sounds like lighter weight Polyester Double Knits and those 1980s polyester jersey variations (what were they called? The knits Diane von Furstenberg used in her wrap dresses) by a new name...
And 'mesh' knits I thought were the rougher type (not the jersey knits) that cotton polo shirts were made of...
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edited to say: Rereading this and thinking more about it, I sound pretty cranky in these posts. I think I have a lot of preconceived notions about polyesters, having experienced them as they were being developed. I do generally like Malden Mills polys. But I have very bad memories of trying to fuse interfacing to poly jersey (and ruining the fabric with the heat) and of being extremely uncomfortable wearing polyester clothing. It could be that I'm just plain allergic, or I had poor quality polys....
I do think that easy-for-beginner-sewers fabrics are a good idea. And that EmmaOneSock/Linda is very creative with her site and has a lot of really nice fabrics. I like that she includes info on the fabrics and sewing them.
I guess I was trying to figure out if these are her name-inventions or if they are standard RTW terminology.
Anyone know?