Fin vs flipper?

What is the difference between fins and flippers?

While my own intuition says that a fin would be fixed (like, a shark's dorsal fin), and a flipper could be moved about way more (like a turtle's flippers), the following wikipedia page suggests that fins may be on fish and flippers on mammals and others. https://en.wikipedia.org/wiki/Fin_and_flipper_locomotion

Both translate to the same word in my native language German.

Which is correct?

The technical difference is that a flipper is a modified limb and a fin is not. So a dolphin has a fin on his back and flippers where a mammal's forelimbs are.

With this, most fish would have only fins, aside from the lungfish which have leggy fins to walk on land. Sea turtles would have flippers.

Flipper (like fluke) is more restricted than fin, which ODO defines as

A flattened appendage on various parts of the body of many aquatic vertebrates, including fish and cetaceans, and some invertebrates, used for propelling, steering, and balancing.

as opposed to flipper,

A broad flat limb without fingers, used for swimming by various sea animals such as seals, whales, and turtles.

The terms flipper and fluke are almost never used to describe parts of fish, even when the appendages are fleshy, as with shark fins, or lobed, as with lungfish, which indeed are known as lobe-finned fish.

For cetaceans, many people use fin and flipper interchangeably, but in more formal discussions of anatomy, references to dorsal fins, pectoral flippers, and tail flukes are most common. Dorsal flippers is far rarer, not turning up at all in a Google Ngram search. Pectoral fins appears to be similarly rare in reference to mammals, although this is difficult to establish, as fish do have pectoral fins, and not only fish but aircraft and Cadillacs have tail fins as well.