If you have a modern western astrological chart, you will see that it is a 360 degree circle with divisions for the 12 signs and another set of 12 divisions known as houses. The passage of the Sun through the 12 signs of the zodiac marks the passage of a year. The passage of the houses through the zodiac are based on the earth's rotation and mark the passage of a day.
The house divisions begin with the Ascendant (Asc). This point is based on the sign that was rising in the East at a precise time, date and location (such as for your birth) and is the beginning point for the first house. The point opposite is called the Descendant (Dsc).
The point which was overhead when you were born is the Midheaven (MC) or culminating point. This is the highest point a planet can reach (as distinguished from the zenith, which is a point directly overhead or 90 degrees from the ascendant). The point opposite is called the Imum coeli (IC).
The 4 angles formed by the Ascendant/Descendant, Midheaven/IC form the base for all house systems.