The name "Flounder" is applied now to so many different (though related for the most part) species of fish that it's on the verge of becoming useless.
In Europe, the species of fish is "Platichthys flesus." In North America, Flounder in general is a saltwater fish living off the east coast. But, there are Southern Flounders (Achiropsettidae) and Summer Flounders (Paralichthys dentatus, aka Fluke, aka Large-tooth Flounders.) Summer Flounders in turn down into Lefteye Flours (Bothidae) and Righteye Flounders (Pleuronectidae.) Some halibuts and plaice are also referred to flounders.
Note that despite all the confusion, Flounder is not sole. Sole is Solea solea and while it does live in the Atlantic, it's the Atlantic off northern Europe. Most of what is labelled "sole" sold in North America is actually Flounder.
But you could say, that given that so many species are now referred to as "Flounder", that the term Flounder has little use left beyond referring to a flat fish of some sort.
Flounder in general are a flat fish with the eyes on top. When the fish are young, the eyes are on each side of the head, as you would expect with a fish. But as the fish starts to grow up, one eye migrates to the other side of its body, and then the fish starts to swim tilted, so that that side with the two eyes becomes the top side. This upper side turns dark; the underside is white.
Flounder will be 5 to 15 inches (12 1/2 to 38 cm) long, though they can occasionally be up to 24 inches (60 cm) long. They have a roundish body: the fish will be about half as wide as it is long.
The fish not only live on the bottom on the sea, but even spend a good deal of time actually buried in the sand. They move by flexing their entire bodies. They eat mussels, insects, and fish hatchlings.
They come close to shore to lay eggs. Each egg has a small oil bubble in it which causes the egg to float, either in vegetation or right up to the surface.
Flounder are not very tasty when caught in the summer; spring and fall-caught ones are best.
When sold whole, they are usually trimmed of head and fins. Larger ones may be sold by fresh or frozen fillets.
When cooked, the flesh is flaky white, and has a mild taste.
Flounder is one of the fish often chosen for Japanese "ikezukuri sashimi" (fish served live on the diner's plate.) The exact species used is Japanese Flounder (Paralichthys olivaceous) -- yet another species of fish referred to as Flounder.
Can be cooked as you would any fish; don't overcook.
Freeze for up to 3 to 4 months.
The Spanish name "Lenguado" is the same for both flounder and sole.