Bathtub ring

These are deposits of water-borne minerals, body oils and soaps – and any salts or perfumes you’ve added to the bath water – that gather at the high-water mark around the surface. Wiping the sides of the tub down after each use will reduce the need for heavier cleaning strategies. Remove heavier build-ups by applying an all-purpose cleaner, allowing it to work for several minutes, then scrubbing gently with a white nylon pad.

Remove any remaining mineral deposits by soaking then scrubbing with a) vinegar, and if that doesn't work b) a properly diluted solution of citric or phosphoric acid, which is an ingredient in many tile and tub cleaners. If you need to resort to this last step, wear protective goggles and gloves and ventilate the area properly, since stronger acids can be eye, skin, and respiratory irritants.

Reference:

Don Aslett in The Cleaning Encyclopedia: Your A to Z Illustrated Guide to Cleaning Like the Pros.