What is CIP rules, descriptors?

The CIP rules (Cahn–Ingold–Prelog) rank the four substituents of a stereogenic centre: higher atomic number first; on a tie, compare the sets of atoms attached next, and so on outwards; a double bond counts as two single bonds to duplicated atoms. With the lowest-ranked substituent pointing away from the eye, the sequence $1 \to 2 \to 3$ turning clockwise defines the descriptor (R), and anticlockwise (S). For a double bond with two substituents on each carbon, (Z) means the two higher-ranked ones are on the same side and (E) on opposite sides.