First, it is used to describe the rear end of an animal's body, especially when it forms a distinct, flexible appendage to the trunk. It is the part of the body that roughly corresponds to the sacrum and coccyx in mammals and birds. Its uses include locomotion (e.g. fish), balance (e.g. cat), grasping (e.g. monkey), social signals (dog), defense. Human embryos have a tail that measures about one-sixth of the size of the embryo itself. As the embryo develops into a fetus, the tail is absorbed by the growing body. Infrequently, a child is born with a "soft tail," which contains no vertebrae, but only blood vessels, muscles, and nerves. Modern procedures allow doctors to eliminate the tail at delivery. The longest human tail on record belonged to a twelve-year-old boy living in what was then Indochina, which measured nine inches.
It can also describe anything like an animal's tail in form or position, such as the tail of a shirt.
The obverse side of a coin is called the tails side.
A long braid or tress of hair is sometimes called a tail, although queue is probably a better description.
A tail can be the rear or back section of an aircraft or missile.
A tuxedo jacket is sometimes referred to as "tails" if the bottom part of the jacket is in the shape of a penguin's backside.
To tail someone in surveillance is to follow that person stealthily.
In the Unix operating system, the tail command provides the last few lines of a file.
In crossword clues the tail is the end of the words. Thus Army Detail is Arm.