The Twelve Houses
The twelve areas of life a natal chart divides into.
What The Houses Add
A sign describes a style and a planet describes a function. A house describes a location - the area of life where that function is actually spent.
A planet in a house says where its function is most visible. Mars in the tenth reads as ambition aimed at a career; the same Mars in the fourth reads as energy poured into a household. Same drive, different address.
This is the layer that turns a general description into a specific one, and it is the reason two people can share every planetary placement by sign and have completely different charts.
The Four That Matter Most
Learn the first, fourth, seventh and tenth before the other eight. These sit at the angles of a chart and carry more weight than the rest: approach, foundation, partnership and public standing.
The first house cusp is the ascendant and the tenth house cusp is the midheaven. Anything sitting in or near those four points does the most visible work in a life.
The Calculation Problem
There are more than a dozen house systems in active use and they produce different answers for the same birth. Placidus divides the time a degree of the ecliptic takes to rise; Equal House marks twelve thirty-degree segments from the ascendant; Whole Sign makes each house exactly one sign.
Near the poles the disagreement becomes severe - Placidus is mathematically undefined above the Arctic and Antarctic circles. This site uses Placidus, the most widely used system, and falls back to equal houses above the polar circles, and says so on the chart rather than quietly filling in.
The practical consequence: a planet near a house cusp may sit differently in another system. Placements in the middle of a house are stable and are the ones worth building an interpretation on.
Empty Houses Are Normal
There are ten planets and twelve houses, so most houses in most charts are empty. A typical chart has five or six.
An empty house is not an empty area of life. It means the chart does not concentrate energy there. Read it through the ruler of the sign on its cusp - find that planet and see where it sits - which is where the actual information is, and which is the step most introductory material skips.