The 12 Signs of the zodiac are shown below in South Indian style [the most logical]. The top left corner is always Pisces/Mina. A circle has 360 degrees. Divided by 12 gives 30 degrees for each Sign.

Pisces Aries Taurus Gemini
Aquarius     Cancer
Capricorn     Leo
Sagittarius Scorpio Libra Virgo

In this south Indian style chart, the signs are always fixed. The ascendant is marked as 1 or with a slanting line.

Mina Mesh Vrsabha Mithuna
Kumbha     Karkata
Makara      Simha 
Dhanus Vrishchika Tula Kanya

Constellation is a group of stars forming a design as seen from earth. A constellation is not exactly 30 degrees. Nor does the group form any particular design. For example the Leo constellation can look like a triangle and a question mark. Then why is this called a Leo? Why not a cow?

Thousands of years ago the names of the zodiac was fixed in Hindu Astrology. In Sanskrit language. By Indian Sages who understood the link between humans and the planets only as a result of the power of their meditation.

Some common properties were found in humans. Such groups of humans had planets in same sign in their horoscope.

For easy rememberance, the constellation names were selected of animals that had these properties. Therefore we have a Lion as Leo Sign.

The same names have been used later by Egyptian and European astrologers for same reason. Astronomers luckily did not change the names to numbers.

The planets go around the sun in almost a level plane. Therefore, the Sun, Moon and other planets form a narrow path in the sky when seen from earth. This path is called the zodiac. The signs are called by the name of the main constellation on it.