Mermaids are mythological creatures, appearing in folklore across the globe for thousands of years. One of the oldest and most well-known myths comes from Ancient Greece. A popular legend claimed that Alexander the Great's sister, Thessalonike, turned into a mermaid upon her death and lived in the Aegean. Whenever a ship passed she would ask the sailors one question: Is King Alexander alive? If they answered correctly, saying "He lives and reigns and conquers the world," she would calm the waters and bid the ship farewell. Any other answer would anger her, and she would stir up a terrible storm, essentially dooming the ship and every sailor on board to a death at sea.
Many tales such as this one equate mermaids to the sirens of Ancient Greek Folklore, who lure men to their deaths on the sea. These legends speak of the unearthly beauty of these sea maidens, which is so captivating that men cannot help but follow them into the sea. This is just one of many tellings of mermaid lore, however. In other traditions, mermaids can be benevolent or beneficent, giving gifts or falling in love with humans. Tales are often recounted in which mermaids save men who have been shipwrecked and bring them safely to shore. In modern times, the most popular mermaid tale is probably Hans Christian Anderson's The Little Mermaid. Unknown to many, however, this story too has a dark history, and doesn't originally end in the "happily ever after" we know from the Disney classic...
To learn more about the history of Mermaid Mythology, see the article on Wikipedia. You might also want to check out Mermaid Legends from Around the World.
If you're interested in Hans Christian Andersen's tale, here's a link to his The Little Mermaid.