A traditional cauldron is a cast iron, pot-bellied pot with three legs. If you cannot find such a thing, any pot will do in a pinch. It represents the element of water, but is also used to house small fires (especially at Beltane). Still water in a cauldron makes for excellent scrying.
Any heat-proof dish or container for burning incense. Censers that hang on chains (Catholic style) are also called 'thuribles'.
Chakras are the 7 major nodes of energy present in the human body. They are located from the crown of the head to the bottom of the torso, lined up along the spine. Each resonates with a different colour, and corresponds to a particular kind of energy. The concept of the chakras is not specifically Wiccan, but comes from Eastern spiritual beliefs.
A chalice (or cup, goblet) is one of the most common altar tools. It represents the element water, and is often used during ritual to hold water or wine. A typical chalice is a stemmed glass, but any shape or material will serve the purpose.
A group of witches that gather for ritual on a somewhat regular basis. Covens usually celebrate the Sabbats and Esbats together, as well as other rituals. A coven does not require 13 members as some may think. Some groups are strictly structured (Alexandrian or Gardnerian) but some are more open and casual.