You can set up an altar any way you want to. The four elements are usually represented somehow. Candles and other objects are usually used to represent them. The God and Goddess can also be represented by the color gold and silver. They can also be presented as the sun and the moon. Altars can be set up for particular events like the Sabbats or specific spirits or something else. They are usually where magic is done with spells and rituals. Altars can be permanent or put up and taken down.