Flyers and sliders - mostly used by match anglers to pick up a few bonus fish by fishing two water levels at the same time.
I would personally call the 'sliding a snood down the mainline' a 'slider'. I always struggled to use a slider effectively so I don't bother now. The 'flyer' on the other hand is very good at picking out fish like mackerel, coalies.
To fish a 'flyer', I thread 2 small beads onto my leader before attaching the swivel/link. Then, if I want to fish a 'flyer' at a shallow mark, i simply clip in a short hooklength in between the beads. When you cast out, the flyer flies back up the line toward the leader knot. A tapered leader is useful when using the flyer simply because it can fly further back up the line away from the main baits - the idea being that you have different baits spread out accross a wide range of sea bed and depth. Add a floaty bead to your flyer hook length to fish in the upper layers of the water, add a small ball weight (and let out some line after the cast) to fish the flyer on the sea bed (good for flounder close in while the main bait is further out)
A picture sure does paint a thousand words, but I hope you get the gist of what I am trying to say