Ariel is a
mermaid that stars in
Disney's 1989 production,
The Little Mermaid. She is a
stubborn teenager (is this phrase
redundant?) and frequently gets into trouble with her
father,
King Triton, who seeks to avoid contact with the human world rather
religiously.
Ariel lives in Atlantica, which is an undersea city constructed from coral and an unknown metallic substance that manages to survive constant exposure to salt water. In Atlantica, pretty much all sea life, from sponges to dolphins to sharks to jellyfish, are sentient and can speak English. This lends rather well to amusing sitautions. Note that Atlantica should not be confused with Atlantis, which was supposedly a human city.
Her favorite pasttime is going up to the surface to watch and study humans, and to collect their paraphernalia, much to her father's annoyance. The human world fascinates her to no end; it's a classic case of "the grass is greener on the other side". This is especially depicted in the TV series, which accounts her adventures before the movie.
All of this obsession eventually culminates in her falling in love with a human, Prince Eric. We don't know the name of the Kingdom Eric is a prince of, and we never see his parents, but all that is inconsequential. Ariel strikes up a dangerous deal with Ursula the Sea Witch to become human, and eventually gets to be with her prince. Awww.
Ariel's voice, in all the productions she's been in, is provided by Jodi Benson. You may have also heard her voice in Barbie commercials and in some other Disney movies.