Floss is typically used to create fly bodies, and, in the case of salmon flies, butts. It can even be used to create wings on salmon flies. Thread is typically used to hold stuff like that down. BUT, having said that, thread can be found in diameters sufficient to use for bodies and butts. There is a seemingly endless supply of both floss and thread in terms of diameters, number of strands, shine, flatness or roundness, waxed or no wax, yada yada yada.
I just did a piece on my blog that highlights the differences in flosses and includes some threads for comparison. I'm not going to re-create that here; if the moderators wish, they can certainly delete this post. No biggie.
One way to think of the difference is to think of floss like a ribbon, wide and thin. It's used, as Gary said, to form bodies on some patterns-- typically old time classic patterns like feather wing streamers, softhackle trout flies and Atlantic Salmon patterns, and some new patterns that resemble the tying in these classic styles.
Traditionally made of silk, floss can also be found in Rayon and other synthetics. Floss can also be a bit fragile and tricky to work with, often requiring a bit of swearing to get uniform wraps and a smooth body. Floss bodies usually get a ribbing of some sort (wire or oval tinsel) to protect it from unravelling as individual strands of floss become broken by fishes teeth.
For some excellent examples of some flies tied with floss bodies, here's a thread from an old Salmon Fly Swap that Pocono ran awhile back: Salmon Fly Swap
Take a look at the beautiful floss bodies in:
Ard's Blue Charm tied by Hardyreels
Miramichi Cosseboom, tied by Aroostockbasser
Alaska Maryanne tied by Pocono
Thread on the other hand is used to bind materials to the hook, and is comparatively easy to work with. Thread is generally round in profile, though it can often be flattened or corded more tightly by spinning the bobbin one way or the other to unwind/wind it. Thread comes in a variety of thicknesses (measured in "denier") a typical size for many trout flies is 70 denier (examples of 70 denier thread include Danville Flymaster 6/0, or Uni 8/0, UTC 70 etc). Note that the "ought rating" ie 6/0, 8/0 12/0 etc varies by manufacturer, with the larger number (say 12/0) being thinner than a lower number (say 8/0), but the comparison only works for threads in the same brand-- for example both Danville 6/0 and Uni 8/0 are 70 denier. In addition to 70 dernier, thread comes in other thicknesses which may be appropriate depending on what you're tying, but it's a good all around choice for most trout sized flies. If you were tying spun deer hair bass bugs which require a lot of thread torque or large saltwater streamers you might want something thicker and stronger.
Let us know what patterns you're tying and we can suggest some materials for you. Or if you already have some materials we can suggest patterns that use them.
I bought a spool of Danville's black acetate floss 'cause I read somewhere that you could wrap and cement ant bodies and the cement would melt the acetate and form neat hard ant bodies. Didn't work, probably the wrong cement. And, man, is that floss hard to work with. It likes to separate and the strands break and the fly gets fuzzy while it's still being built.
Wow, those are some beautiful patterns, some of you guys really have some skill. I can't think of any patterns off of the top of my head, but I was looking mostly at dry fly and nymph patterns and only ever saw thread in the recipe, never floss, but I kept seeing floss advertised.
you have to "wiggle" the floss to keep the strands together... and keep in mind as you WIND IT, it either increases or undoes the 'twists' of the material which keep the strands together.
Another tip with floss, if you have rough fingertips, you should use an emery board on them before tying with floss, wash your hands thoroughly, and make sure you don't have any lotion or creams on them. In extreme cases, you can buy Nu-Skin and paint it on your fingertips to form a coating that won't fray floss- if you're tying dress display salmon flies or planning a long tying session with floss.
And if you're tying on a black or bronze hook, you may want to consider an under layer of white thread or silver tinsel to ensure the floss ends up the color you wanted it to be in the first place when you fish the fly... when most floss gets wet, it becomes translucent and can darken as well... which is where you may want to consider a coat of lacquer over the top of it. I've also wound over the top of floss with clear sewing thread (essentially monofilament, thinner than 8x)
That's exactly what I was getting about about the base material Gary. I learned this bout 40 yrs ago with dry/wet flies I tied with floss bodies... WAY before mylar tinsel =)
Floss bobbins are okay with single strand floss or even multi strand *IF* you use all 2/4 strands on the bodies. Unfortunately, many of the flies I tie involve one strand of 4 strand floss, so I'm left to bare handing it. I've used 'finger cots' at times as well, especially if I tie in the field when I need to replenish a supply of flies, where I'm less likely to be able to scrub and scour my hands.
You'll probably want to tie some stuff with floss at some point, but if you're just starting out there are many other options of stuff that is easier to work with. And if you do want to try some floss you could start with something like Uni Stretch Floss-- a synthetic that's easier to work with than silk.
As far as patterns that use floss, probably the most common dry fly would be the Royal Wulff, where there's a red floss band between the 2 sections of peacock herl.
Classic Soft Hackle wet flies like the Partridge and Orange (partridge and yellow, partridge and green etc) also traditionally use silk floss, but again there are many other choices including thread, synthetics, dubbing etc that you can use for bodies on soft hackles and other wet flies.
Silk floss does have some interesting properties in addition to the link it has to its long heritage with fly tying over the centuries. For one, it becomes translucent when wet--On a silk body over a bare hook the metal shank can give the fly a translucent glow through the wet silk. And here's a great thread from Joni showing a blue silk floss underbody and yellow silk floss overbody she used on a Chironomid (midge) pupa pattern. Just add water and voila! More Silk
For trout flies, some excellent patterns to start if you're new to tying would be the Woolly Bugger, Pheasant Tail Nymph and Gold Ribbed Hare's Ear Nymph. They catch fish, are relatively easy to learn, use inexpensive materials that can be used on a lot of other patterns, and teach core skills that you'll use on a lot of different patterns.
Keep asking questions as you get into it... Materials can be very confusing and hopefully we can help a bit.