The cauldron represents Akasha. It also symbolizes the transformation point that happens within the magickal process. It is placed within the center of the circle when used in ritual. The reason being that it is also known as the "Womb of the Goddess". In other words the source from which all magick flows.

In ancient times a cauldron was nearly always constructed of cast iron as it held fire. Nowadays of course, that "fire" usually takes the form of incense, paper requests, smoldering herbs, candles etc. so almost any metal will do. Cast iron is still preferable though.