Saju (사주, 四柱) means "Four Pillars." Its fuller name, Saju Palja (사주팔자, "Four Pillars, Eight Characters"), holds the whole idea in one phrase: your birth moment is written as four pillars — the year, month, day, and hour you were born — and each pillar carries two characters, for eight in total.
Each pillar pairs a Heavenly Stem on top with an Earthly Branch below. Between them they encode one of the five elements — Wood, Fire, Earth, Metal, and Water — and whether it runs yang (active) or yin (receptive). Read together, the eight characters describe the balance of elements you were born into.
The Day Master — the "you" of the chart
Of the eight characters, one is the anchor: the stem of your day pillar, called the Day Master (일간, il-gan). It stands for the self, and everything else in the chart is read in relation to it — which elements support it, which pull against it, and how today's energy meets it. If you already read Western astrology, the Day Master is the closest thing to your sun sign: the core of who you are.
Saju is not the Western zodiac
Western astrology is built from the positions of the sun, moon, and planets. Saju is built from a calendar of cyclical elements — no planets, no constellations, and no twelve "signs." Where a sun sign comes from the month you were born, your Saju is drawn from your whole birth moment, which is why the hour, when you know it, sharpens the reading.
The five elements, in motion
The heart of a reading is how the five elements interact. They generate one another in a cycle — Wood feeds Fire, Fire leaves Earth as ash, Earth bears Metal, Metal carries Water, Water grows Wood — and they control one another in another — Wood parts Earth, Earth dams Water, Water quenches Fire, Fire melts Metal, Metal cuts Wood. A chart where one element floods and another runs dry reads very differently from a balanced one.
That is the whole engine: four pillars, eight characters, five elements, two cycles. A reading is simply a careful description of how your particular balance tends to move through life.