Rainscreen systems and curtain walling are both non-structural cladding for the external walls of buildings.
Curtain walls usually make up the entire building envelope, which is the physical separator between the conditioned and unconditioned environment of a building. Rainscreen cladding is just the outer protective layer of the envelope.
While curtain walling is usually made out of glass panes slotted within an aluminium frame, cladding can be made from a wide array of different materials.