It is said that a mermaid is a very attractive woman on the top half, and her lower half is that of a fish. One particular story that I read has a mermaid being able to change into a "full" human in order to attend a market so that she could buy things.
The typical image we have for a mermaid is of a beautiful half woman-half fish sitting on a rock, out at sea, combing her hair and or singing. Apparently the mermaids have no souls and lure fishermen to their death, by singing beautiful, haunting songs. When the sailor gets close enough the mermaid grabs him and drowns him. There are also tales that have the mermaids keeping the souls of the fishermen in cages, deep under the sea.
Typically the mermaids are the same size as a typical woman, but some tales have them up to 2000 feet in length.