Mermaids and sirens are said to be half-human, half fish. Male mermaids are known as "mermen".  The difference between a siren and mermaid is that sirens are also part bird and some sources say that they are just part woman, part bird. Mermaids and sirens are said to lure sailors with their singing onto rocks and also drowning them in the process or luring them into the water (both methods end up with death of said sailor). 

One legend tells of a captain who tied himself to a mast and got his crew to put wax in their ears. He commanded his crew (before they plugged their ears) to sail to the location that the sighting are reported at. He is the only person to allegedly hear the siren' s song and live to tell the tale.

Mermaids exist in fairy tales (or Fairy Tails XD), myths, legends and tales around the world.