Whereas the subdivision of the zodiac into four elements is easy to understand, the qualities are harder to make sense of. Indeed, some astrologers feel that they cloud, rather than clarify, the essential characteristics of the signs. They are important, however, because they enable a further sub-division of the elements, so that each of the twelve signs has a unique combination of element and quality.
The extent to which someone will demonstrate these characteristics will depend on the number of planets - and the importance of these planets - in the relevant signs.
The Cardinal signs are Aries, Cancer, Libra and Capricorn. Cardinal signs are very good at starting things, but not quite so good at continuing their enthusiam for any length of time. The emphasis is on intiation and creation, rather than on continuity. This is especially true of Aries and less so of the others.
The fixed signs are Taurus, Leo, Scorpio and Aquarius. These signs are better at continuing change than at initiating it. In the face of change from outside, they will tend to resist it head-on, rather than to try to avoid it. This is especially clear in the case of Taurus, which is notably very stubborn, but is less obvious for the other signs, especially Aquarius.
The mutable signs are Gemini, Virgo, Sagittarius and Pisces. These signs are flexible: instead of actively resisting outside pressure, they try to alter it indirectly by bending the course of events in the direction that they would like it to go. Unlike the fixed signs, which resist change, they adapt it; and unlike the cardinal signs, which initiate change, they take advantage of changes initiated by other people.