: a platform or table used as a center of worship
: a usually raised place on which sacrifices are offered
I got the above easily from google. Depending who or what you worship or honor depends on the items you choose to include on your altar; candles, incense, statues, crosses, pentagrams, etc.
I hope this answered your question.
It a place of worship it can really be on any flat surface. you may have to do some research, because some of the things u will need deepened on if your going to be worshiping a god of goddess. but the most common things to have are of course candles most Wicca and pagan rituals will involve them. Incents are a good thing to have along with herbs such as sage that will go along with you rituals. also if u want to include crystals they help with grounding. lastly having moon water or holy water is a good thing to have to bless your rituals and for protection, if you have any questions on how to make ether of though ill be willing to help. also if you would like a visual aid i can send you a pic of what mine looks like and explain what i have on it and there uses.