The Qualities of astrology refer to three basic modes of operation:
Cardinal, Fixed, and Mutable.

They are also called Modalities and Quadruplicities.  Each mode of expression operates on the same vibrational level in every fourth sign, which form stressful square aspects to each other.

The word "cardinal" relates to anything that is of prime importance.  The cardinal signs are associated with starting new things.  In nature, each of these four signs serve as the beginning of a new season.

During Aries, we experience the Spring Equinox, when the Sun is over the equator on its way north.  In Cancer, we have the Summer Solstice when the Sun has reached its most northerly point and is turning back southward. 

Libra gives us the Autumn Equinox when the Sun is again at the equator, but still heading south.  And Capricorn covers the Winter Solstice when the Sun is as far south as it will go before once again heading north

The four cardinal signs are associated with an urge to get started on things, to initiate change. They don't always finish what they start, but at least they get us moving. Whenever there are a lot of planets in these signs, you will not feel satisfied with the status quo. You will burst at the seams until you start something new.

These are the signs that happen in the middle of the season, when the essence of the season is at its fullest.  They are not being pulled toward either end of the season when things change.  And, as the name of this quality  implies, they do not accept change very well.

These signs relate to all that is solid and stable -- the earth, our egos, our deepest secrets, what we "know". 

They are associated with dependability, stability, and deep concentration.  When a lot of planets are in fixed signs, you will find yourself standing your ground, right or wrong, and just plowing on through.

The keyword here is variety. As the last sign of each season, these signs represent times when we are finishing up one set of activities and planning for a new set.  As such, they are associated with being able to handle a wide variety of things.  In fact, they need a wide variety of things to feel complete.

All of these signs thirst for more -- more ideas, more details, more knowledge, and more love.

These are the ones who can easily adapt to change, who in fact love it!  A heavy emphasis on mutable signs is like a meeting of the United Nations all talking at once.