Your birth chart is divided into 12 houses, each ruling an area of life — from your identity (1st) to your career (10th) to your inner world (12th). Every planet lands in one house, showing where its energy expresses itself. This encyclopedia covers all 144 planet-in-house combinations, so you can look up any placement and understand what it says about your life.
Each house is a department of life: the 1st is identity and body, the 2nd money and values, the 4th home and family, the 7th partnership, the 10th career and reputation, the 12th the unconscious and retreat. The list runs counter-clockwise from your rising sign, so the house meanings stay fixed while the signs attached to them move.
Yes — more than for anything else in astrology. The house cusps move roughly one degree every four minutes, so a birth time that is an hour off can shift placements by a whole house. Without a reliable time, read planets in signs instead and treat the houses as an approximation.
Because it decides where the energy lands. A planet in the 10th house shows up in your public and professional life whatever sign it is in; the same planet in the 4th plays out at home. Sign describes the style, house describes the arena — and the arena is usually what you notice first.