Groups of stars
If you ask someone what constellation is, he/she will probably answer you that it is the group of stars located in certain form after which the constellation got its name. And this person will be completely right in the 19th century.
Nowadays scientists claim that constellation is the area in the sky that unites special group of stars that have special order. They decided to take such definition to make the work easier because some stars entered several constellations and there were also stars that didn't belong to any constellation. Such confusion made scientist think this problem over. In 1922 the General Assembly of the International Association of Astronomers made the final list of 88 constellations into which the sky was divided. And in eight years the exact and final limits between them were made. During five years scientists corrected some limits. But in 1935 limits were adapted finally and couldn't be changed any more.
Only 47 of 88 constellations are the ancient ones that are known to people millenniums already. They are usually based on the mythology of Ancient Greece and include the area of the sky that is available for watching from the South of Europe. Other modern constellations were introduced in 17-18th centuries in the result of investigation of the Southern sky and filling the gaps in the Northern sky. The names of these constellations usually don't have mythological origins.
There are also traditional 12 constellations that make 12 signs of zodiac. These constellations are those through which the sun comes. There is also another 13th zodiac. But the sun doesn't pass even several days in this zodiac that is why it is not included into the traditional range of zodiacs.