Polishing is to bring back the paint surface to its original smooth, shinny and easy to maintain finish. It can also remove oxidation, road grime, fine scratches, swirl marks, and water stains. Recommended SCG polish: Beyond Clay.
Sealant or Wax is a topping that adds brilliant shine and protection to paint surface. It should be applied after polishing. Regular waxing maintains a nice shine on your car and helps to protect the paint. Recommended SCG sealant and wax: Shine-On Sealer & Glaze System / Barrier Reef Carnauba Wax.
Some products (also known as All-In-One) contain both polish and wax so not only do they clean the paintwork, they leave a protective layer of wax too.