Fishtail tools have a shank that narrows rapidly from the cutting edge, so that the corner of the cutting edge is less than 90 degrees. This makes cutting into corners, especially on relief carving much easier. Fishtail gouges are marked with a "F" after the sweep code ex. 3F/20 (#3 Fishtail gouge, 20 mm wide.)
Here is a comparison between a regular gouge and a fishtail:
Here is a comparison between a regular gouge and a fishtail: