In the Disney version, the film ends with Ariel the mermaid being changed into a human so she can marry Eric. They marry in a wonderful wedding attended by humans and merpeople.
In the original version by Hans Christian Andersen, the mermaid sees
the Prince marry a princess and she despairs. She is offered a knife
with which to stab the prince to death, but rather than do that she
jumps into the sea and dies by turning to froth.
Andersen later modified the ending to make it more pleasant. In the
new ending, instead of dying when turned to froth, she becomes a
'daughter of the air' waiting to go to heaven.