There are two kind of altars. Devotional altars and functional altars.
A devotional altar is a sacred space dedicated entirely to one or many deities. There is usually a figurine or image representing that deity. In it you place the offerings, prayers and stuff for that deity. It can be simple or heavily ornameted. Ornaments can be permanent, or can be changed according to festivities and specific moments during the year.
I don’t have a devotional altar. The only thing that’s worth my devotion is myself, and my body and mind are my devotional altar. I take care of myself.
Then there’s the functional altar: this is the altar in which you do all the rituals and spiritual/energy workings. In it you place your tools. The tools can also represent a plethora of things, like elemental energies or specific concepts. Here’s a photo of my functional altar:
I don’t go about the usual symbolism of the tools:
The athame (ritual dagger) symbolises air, but more importantly, it is a symbol of direction, of attack, of an action that has a specific target to it.
The chalice symbolises water, but also is a symbol of containing, memory, knowledge and wisdom. It’s a recipient that’s constantly learning, and can never be filled. It’s by proxy a symbol of the mind.
The mortar and pestle is a symbol of earth, but more importantly, it’s a symbol of alchemy, of mixing, experimentation, the natural cycle, curiosity.
The candles represent fire, but also light, transformation, energy, rebirth, destruction. To be consumed by fire means rebirth into a new form.
Then there’s the pendulum and the tarot cards. The pendulum is a symbol of duality, of two poles with an infinity of nuances in between.
The stones at the top are a Quartz (symbol of light, of that which is transparent, clear, easy to see), Lapiz Lazuli (it’s a symbol of myself), and Obsidian (symbol of that which is hidden, the truth that has to be throughly searched for in order to be found).