While specific traditions may have certain requirements of how an altar should be set up, generally speaking altars come in all sorts of shapes and sizes, and can serve many different functions. They can be outdoors or indoors, in any room of the house (yes, even a bathroom might house a small purification altar, or an altar devoted to water in the shower). Altars can be big, small, formal, informal, hold only a few items, or be covered in things, they can be obviously altars, or quietly fit into their surroundings. The altar may be kept in a busy spot in the home, where it will be seen often and always be bringing your faith to the front of your mind. Or it can be kept in a more quiet location, for privacy, or to keep it from being bumped, touched, or whatever else. An altar can be out in the open, or kept in a small cuppboard. It can be a permanent altar, or one that is only set up when needed. An altar can change with the seasons, can be devoted to a deity, a place to honor ancestors or spirits, a place to do ritual workings, a place to meditate, make offerings - many will be a mix of a few things.
Basically, what I'm saying is, what an altar is? That's up to you. You may choose to not even have one, and that's fine too if that's what works for you. Personally though, I keep a few different altars in my home...
Artemis' (indoor) altar is fairly clean and simple. A statue of her, two tall tea light holders, and two small offering bowls. Usually one for food, and one for liquid, but occasionally I will put some salt into one of the bowls and use it to burn an offering of incense. Sometimes the altar is decorated with flowers, evergreens, or other plants, but most of the time it's just as it's shown. This altar is in the living room.
Then there's the hearth altar, kept in a corner of my kitchen. The top section is devoted to Westya, a hearth goddess. The candle's flame represents her, a symbol of the hearth fire. The candle sits in a small bowl that has some non-perishable offerings in it. Those offerings get changed each new moon, and are usually related to the season. Ground corn around Lammas, or pumpkin seeds at Samhain, for example. The two little plates are more offering bowls, for the daily devotion done at this altar. In front is a smooth flat stone, to be used in spirit-flight rituals as a hearth stone, a stone to help one come back from journeying.
Below that is not really an altar, but more of a little working space of odds and ends. Mortar and pestle, candle snuffer, shell filled with bits (such as a piece of sea glass, a penny, a key...), bowl of bits (such as birch bark, dog hair, a feather...), blessed candles, ritual oil holder, salve holder, a charm of berries, some stones... This space changes often.
Then on the next down is the ancestor's altar. A statue of Yinepu (Anubis), a deity of the dead. Many of my ancestor items are currently packed away from when I re-arranged my altars, I need to get them back out sometime... Meanwhile, the main points of the altar are two offering bowls for the ancestors, and one for Yinepu. (Below that are two more shelves, storage areas.)
Next to this altar is a cabinet where I keep my tools and some supplies, like candles, incense, oils, and the like. Some people like to keep their tools on their altars, but since these areas are rather small, I keep them safely in the cabinet when not in use.
Finally, there's the newest of the bunch. Just set up a few days ago, actually. Hidden away in the corner for a bit of private space, low to the ground so I can sit in front of it. Just a little working place, a place to meditate, do shamanic work, small rituals, and so on. To the left of the altar is a sistrum (a sort of rattle), on the top is a little wooden frog noise maker, incense, a beeswax candle with some moonstones and a sunstone - all for Artemis, then a few different objects for the elements.
The row below that is still a bit empty, but on the left is a box of small ogham staves, in the center I'm working on a space to honor my spirit helpers, and to the right is where I'm currently keeping my wildwood cards (and where I will likely keep a small area for my 'weekly' card study). Below is just some storage right now, a journal, a shawl, and a box of supplies. I plan to be doing the bulk of my indoor work at the newest altar, so I am excited to see how it will grow and change as time goes on.
Beyond the altars shown here, I also maintain a few outdoor altars (yes, I really like altars...), but I think I've rambled on long enough for now, so those are probably best left to another upcoming project post.
(Altars are also the prompt of the week over at Pagan Prompts.)