Hey "Undecided" 
Welcome to Dragons Den, I'm glad you found us.
OK so what is an altar, an altar can be any stand like a table, corner table etc, it's really just something you can place your Altar Goodies on, it can be a permanent fixture in your home or a simply a table you set up every time you need to use it.
I'm guessing your next question is what do you put on an altar, well a very basic altar would normally have some incense on it, an incense burner, a candle and candle holder, a water vessel, your book of shadows, and a pentacle, the pentacle can be a anything from a nice amulet in the shape of a pentacle to a drawing on the table itself, of course an altar can also be much more elaborate.
Hope that helps
Ammon