The 12 Signs - Celestial House System

The original word for "sign", in greek, is zoidon, and it makes reference to the segments that result from dividing the eclyptic (the apparent orbit of the Sun around the earth) by 30°. 

These 12 exact segments are what today are known as the signs of the zodiac. The names given to this segments, were based upon the perception on the anthropomorphic "images" formed by the constellations that occupy them.

Aries - Taurus - Gemini - Cancer - Leo - Virgo - Libra - Scorpio

Sagittarius - Capricorn - Aquarius - Pisces

  

The purpose of the signs, is to envelop the planets found in their confines, with different traits, and act as mediators between each planet. This means that the signs are the ones that influence the planets and determine their relation to other planets.

Now each sign posseses a number of characteristics unique to each sign, yet most of them share similar traits, so the ancient greeks divided them into many group families. 

The most important one of these divisions, is the one based on the Aristotelic elements: the Triplicity.
  

•  Fire Signs: Aries - Leo - Sagittarius

•  Earth Signs: Taurus - Virgo - Capricorn

•  Air Signs: Gemini - Libra - Aquarius

•  Water Signs: Cancer - Scorpio - Pisces

 The meaning behind the aristotelic elements, is to group the signs based on the qualities of such elements: Hot - Cold - Moist -Dry, with each having 2 of this qualities. So:

- Fire: Hot and Dry
- Earth: Cold and Dry
- Air: Hot and Moist
- Water: Cold and Moist

So a planet placed in, lets say Leo, could be considered to have "Hot and Dry" along with its natural traits.

Most of the planets falling into signs of similar Triplicity, share many same traits in common with each other. In a sense, you can say they "get along well". They identify with each other, so the relation between the planets occurs without trouble.

Planets that fall into signs of the same temperature, also get along reasonably well, because they share at least one trait in common (for example Earth and Water, being both cold, but one dry and the other moist).

Planets that find themselves in signs with different temperatures, tend to get along poorly, because they share no affinity (as in Fire and Water).

This is the biggest and most important purpose of the signs. There are many more ways to classify the signs based on different traits, forming many more families. But putting up a list with ALL of these would take for ever!