more usually seen as being a place for magic working. A witch who practises religion may well keep statues or items of religious significance on there.
An altar will likely have some representation of the God and Goddess on it. Witches who work with personal deities may choose to depict a deity from a specific pantheon, such as Hermes the Hunter or Athena the Greek Goddess of Wisdom. Non religious witches may choose to honour personal spirits or ancestors or elementals they work with.
A chalice or cauldron is often used to symbolise the feminine and an atheme or wand may be used to represent the God. Usually the left side is dedicated to the Goddess and the right side dedicated to the God. The four elements may be represented on the altar. Common symbols would include a feather to represent air, a bowl of water for water, incense for fire and a crystal or a rock for earth. These are fairly typical Wiccan structures, a non Wiccan witch may well choose either not to have an altar or to have an area of personal significance with little formal structure. Basically, the altar can be as individual as the witch.
My own personal altar changes with the sabbats so I refresh it eight times a year. I usually set my altar up as a formal area of worship and I do observe the symbols of God and Goddess, though I do not always depict the four elements. I use my altar throughout the week to burn candles for healing or for doing visualisation work. I may use it to charge crystals if they need extra power. In my formal weekly ritual I may charge my tools on the altar, I will likely light candles to honour the God and Goddess and I will pray before it and offer my dedication to my deities.
Picture is my altar at the current time Mabon-Samhain.