Many people use altars in the lives. An altar helps to set your focus for rituals, and magic workings. They can be used to honour deity, spirits and ancestors, and do magic. Many of times however an altar isn't only used for those purposes, it is mostly used to show a symbol of your faith.
Things that can be on the altar as long as some things that should be on it are :
candles (typically candles to symbolize the gods, working candle, and elements)
Altar cloths (some have only one, while others have different ones. It's up to you)
bowls (this can hold the salt and water if used)
Mixing bowl or something to crush herbs and other things. (Good idea for people who use teas, herbs, and food items)
Your books on the craft/faith
anything that is special to you. I have a religion book on mine, and a bible because I hold the belief that religion is special and has something to teach no matter what faith you are.
Different types of altars?
There can be portable altars, which I have one. There can be permanent ones as well. They can be small(if it's only to honour deity) and they can be big. Anything can be an altar really.
Some links :
Any more information to add? Feel free to post below or message me using the chatter. Add any additional links or books if wanting.