What is it? Nylon upholstery thread. So, it's tough and color fast. S-Lon and c-lon are the same product; they're just branded differently by the suppliers.
Sizes: It's available is a number of sizes. As a jeweler I think of it as "thread" and "cord". The thread sizes are AA and D, very fine and frequently used by beadweavers. I use it when I make elaborate tassels and to whip the ends of braids, etc. The cord is currently available in 4 sizes, with the Tex 210 being the "standard". It's a little less than 1mm in diameter and is my "go to" size. The Tex 400 is heavier, a little greater than 1mm in diameter and is great for both kumihimo and macrame. The Tex 135 is referred to as "fine" and is what I use when I have beads with really small holes (pearls, size 11/o seeds, etc). The last is referred to as Micro-cord and is great for beadweavers who must make multiple passes through a single bead.
I get lots of questions about seed beads. Sadly the answers are all over the place. A single manufacturer will have variances in the same size bead from color to color. A good rule of thumb for a beadweaver is to use the same size from the same manufacturer in a single piece. Since the techniques I mostly use are kumihimo, macrame, and fingerweaving I tend to like texture in my pieces so I work solely from color. Mixing manufacturers works just fine for me.
Now with that said here are some specifics:
Seed beads are sized by the number of beads/inch. Hence a size 11/o seed is much smaller than a size 6/o seed and size 2/o is larger than a 6/o. a 6/o seed bead should not be confused with a 6mm bead.
There are a few primary manufacturers of seeds and they all have pros and cons. The three major suppliers from Japan are Miyuki, Toho, and Matsuno with Miyuki being by far the leader in the market.
Miyuki: known for consistency in sizing (vs Matsuno) this brand offers by far the largest range of colors, shapes and finishes.
Matsuno: Much less expensive than Miyuki but sizes are uneven and colors/shapes are limited.
Toho: Generally the most expensive of the Japanese beads, but these are favorites of mine. They have a much larger hole to wall ratio than the other two and the special finishes are just that - "special". The larger hole makes them especially good for working with larger textiles such as ribbon.
All Japanese seed beads are loose and are sold by the gram weight or the tube size.
Czech Seed Beads:
Sadly this industry has been in turmoil since World War II. Once THE major force in seed beads the industry has fallen behind in the past several years. Jablonex and Ornelia have both been purchased by Precosia and Precosia seems to be concentrating on crystals and pressed glass over seed beads. This is really unfortunate because these beads have the smallest hole to wall ratio which made them perfect for kumihimo.
Some Czech seeds are loose and sold by the gram weight or the tube size but many are temporarily strung and sold by the "hank" or "half hank". A hank = twelve 20" strands of beads or about 60 grams. The gram weight of all seed beads, regardless of manufacture, will vary based upon the content of the bead.