Do I need both a shower curtain and a liner?

It depends on your curtain material. Fabric curtains (cotton, linen, untreated polyester) need a waterproof liner inside to prevent water from soaking through. PEVA/EVA liners and water resistant treated polyester curtains can work alone. For the best setup: use a decorative fabric curtain outside for aesthetics and a PEVA liner inside for waterproofing.