A hexagram (卦, guà) is a figure of six stacked lines. Each line is either yang, solid, or yin, broken (see yin and yang). Since each of the six can go two ways, there are 2 × 2 × 2 × 2 × 2 × 2 = 64 hexagrams, no more and no less.
How to read it
Lines are counted from the bottom up. The first line is at the bottom, and it's also the first one you cast; the sixth is at the top. Each place has its own meaning, explained in line positions.
Every hexagram is made of two of the trigram figures, one below and one above. For example, Well-Being has Earth above and Heaven below: the heavy sinks, the light rises, and the two meet.
What each hexagram carries
In the classic, each of the 64 has a name, a judgment, an image and six line texts. In Cambios, each hexagram also has two layers: the practical one, which tells you what it means for your question, and the poetic one, closer to the old voice.
The hexagram in a cast
What you cast is your primary hexagram, the situation as it stands. If there are moving lines, a second hexagram appears, the relating hexagram, showing where things are heading.
The numbers 1 to 64 follow the received order, the King Wen sequence. To read one in full, start with How to Read a Hexagram.