The Menger sponge
is a fractal.
It is a cube built up by smaller cubes and where a regular pattern of smaller cubes are removed.
Each face of the sponge is a Sierpinski carpet,
which is why the Menger sponge is sometimes also called a Sierpinski sponge.
At each recursion step, the cubes with an edge on the edge of the larger cube from the previous iteration are kept.

A Sierpinski-Menger snowflake is a variation to the Menger sponge.
It is also a cube-based fractal but now eight corner cubes and the central cube are kept at each recursion step.
This fractal can be thought of as a 3D analog of the Vicsek fractal.

This JavaScript web application animates a rotating 3D Menger sponge or flake.
As a variation, you have the option to change the cubes into spheres.