A cube is a square moved through space, so it gets thicker. All cubes have six sides - the top, the bottom, and four sides. Each of these sides is a plane. In this picture you can see the top and two of the sides. The bottom and the other two sides are hidden.
A cube has eight corners, all of them right angles. You can see seven of the eight corners in this picture.
Our word "cube" comes from the Arabic word "k'ab", meaning cube (like the Ka'aba in Mecca). This is because Arab mathematicians did a lot of early geometry work, and European mathematicians learned it from them.
To find out more about geometry, check out these books from Amazon.com or from your library: