An altar is a small stand where ritual or magic items are placed. This can be a desk or a table, or an altar that you purchased online or elsewhere. They are used in rituals and many circle casting spells. They're also dedicated to deities, and your altar can change depending on what pagan holidays you celebrate.
Typically, an altar has a few stationary items placed upon it. Although what you put on your altar is entirely up to you, here are some things you would typically find.
For Basic Altars
Basic altars consist of a few small items to enhance energy or worship.
Altar cloth. (These are just cloths to cover your altar. usually they have certain colors or symbols that hold meaning, but they can be purely decorative as well!)
Candles. (Usually they're colored stick candles. There are a lot of wonderful articles on candle magic that tell you what different candles mean!)
Incense. (Incense are a lovely way to use aromatherapy to focus your energy. They're also used in many rituals and spells, and there's also some great articles on the use of them!)
Pictures. (Pictures are a great way to honor deities. If you don't have a deity, a picture of the God and Goddess are also good! But remember that whatever you put on your altar is up to you and should be totally personalized to what makes your energy flow)
Those are some of the most basic things to put on your altar. While decorating your altar can be fun and rewarding, try not to go overboard. Overcrowding your altar with items can lead to a cluttered energy which isn't good for any spell, ritual, or prayer.
For More Advanced Altars
These altars typically contain things that are regularly used in more difficult spells or rituals.
Athame. (An athame is a ritualistic dagger. I don't own one myself, but if you have a free roaming familiar be sure to keep yours sheathed)
Altar bells. (Altar bells are small bells that emit a beautiful sound. I have only seen them used in prayer, but don't limit the possibilities to just that!)
Charged items. (Charged items like crystals or jewelry can help enhance the power and energy that flows through your altar space.)
And last but not least for the specifics, a Cauldron. (This may seem like a sterotypical "TV witch" item, but many pagan holiday rituals call for one.)
Now, keep in mind that your altar is to be kept clean at all times. If you need to remove some of your items and store them elsewhere depending on the function of your altar, that's okay!
It's better to have it and not be using it then have a messy energy in your altar space.
Setting Your Altar Up
Now that you have your altar items, setting one up is fairly easy. Symmetrical altar designs are usually most appealing to the eye and therefore create the best energy and focus, but it really just depends on what makes you feel best! Certain rituals or spells can call for specific placements, but this is really just the basic form of your altar.
The Use of an Altar
As I explained before, altars are used in spells, ritual, and worship. Many pagan holidays such as Samhain and Yule have rituals that require many of the items I listed above to be placed upon an altar and used. Instead of thinking of your altar as an object, think of it as an extension of yourself. Feel your energy flow and collect there.
Keeping your altar clean is extremely important. Along with making sure it isn't cluttered with items, make sure that you regularly clear the air there. This can be done by using a smudge stick, sage, or purifying and banishing spells. This keeps old and negative energy out of your sacred space.
It's optional, but I reccomend taking time to meditate and pray at your altar. You become more in tune with it's power, and it just improves your altar using experience by far.