These are 4 essential tools known as the 4 Treasures of the Study [文房四宝 wen fang si bao]. Other useful tools include paperweights, brush holders and deskpads (or you can use newspapers as they absorb the ink pretty well). Most calligraphers and painters will also have their own personal seals to mark their work.
The quintessential tool used to write the characters. The bristles are usually made from animal hair, and Ang Hoon Seng has one (a gift) made of peacock feathers!
In the past, ink sticks were carefully grinded in the inkstone with just the right amount of water to create the perfect consistency so the ink can be absorbed by the rice paper without any bleeding. This is itself is an art. However, these days, premixed ink is readily available and suitable for practice.
Inkstones are used to hold the ink, and usually has a rim from which excess ink can be removed from the brush. Those of better quality are often intricately carved, and are collectibles worth thousands of dollars.
Rice paper is the most commonly used form of medium. They come in a variety of quality, and each absorbs ink differently. Ang Hoon Seng often experiments with unusual medium such as wall paper, papyrus, cloth and many others.