There are eight planets and two lights (the Sun and Moon). Each of these planets moves through the twelve constellations at their own speed depending on their individual orbits. At the moment of our birth they are located in different signs.
For example, when we say that a person is a Libra, on their date of birth the Sun was located in the constellation of Libra. Our natal astrological chart pinpoints where each of these planets and lights were located at the moment of our birth.
The alignment of the planets at the time of your birth dictates what is going on at the time of your birth and consequently what you are going to want out of life? The following is a brief description of each of the planets and what they mean for you.
Sun: How do I need to express my individuality and uniqueness?
Moon: Why did my soul create this Mother for me in this life?
Mercury: How do I receive information from the world?
Venus: What do I love?
Mars: What actions do I need to take?
Jupiter: How do I get to know my spiritual teacher?
Saturn: Why did my soul create this Father for me in this life?
Uranus: How do I find my independent self?
Neptune: What illusion causes me suffering?
Pluto: How ca I describe my shadow self?
The other factor affecting this alignment is which house you are born into, which depends on the exact time of your birth. There is a better explanation of the house idea on this site.