As you know, the 12 Zodiac signs all have personalities. A further way to explore commonalities between the signs is to examine their underlying "Qualities": Cardinal, Fixed, and Mutable. These Quadruplicities, groups of four signs 90 degrees away from each other that reflect a person's attitudes in life and the way they approach projects.
The Cardinal signs are Aries, Cancer, Libra, and Capricorn. They're the ones found at East, North, West, and South - the 1st, 4th, 7th, and 10th houses. Each one has a strong character. They have initiative and motivation, they get things moving, but they're not necessarily good at follow-through.
The Fixed signs are Taurus, Leo, Scorpio, and Aquarius, the 2nd, 5th, 8th, and 11th houses. Because they're stable and reliable, they're the ones who see projects through to completion. However, they can also be stubborn and inflexible.
The Mutable signs are Gemini, Virgo, Sagittarius, and Pisces, the 3rd, 6th, 9th, and 12th houses. These people are happy to fill assigned roles, and are adaptable to changing circumstances. Their weakness is that they can be wishy-washy.