From The Art and Popular Culture Encyclopedia
A mermaid (from the Middle English mere in the obsolete sense 'sea' (as in maritime, the Latin mare, "sea") + maid(en)) is a legendary aquatic creature with the head and torso of human female and the tail of a fish. The male version of a mermaid is called a merman; gender-neutral plurals could be merpeople or merfolk. Various cultures throughout the world have similar figures.
Much like sirens, mermaids in stories would sometimes sing to sailors and enchant them, distracting them from their work and causing them to walk off the deck or cause shipwrecks. Other stories would have them squeeze the life out of drowning men while trying to rescue them. They are also said to take them down to their underwater kingdoms. In Hans Christian Andersen's The Little Mermaid it is said that they forget that humans cannot breathe underwater, while others say they drown men out of spite.
The Sirens of Greek mythology are sometimes portrayed in later folklore as mermaid-like; in fact, some languages use the same word for both creatures. Other related types of mythical or legendary creature are water fairies (e.g. various water nymphs) and selkies, animals that can transform themselves from seals to humans.