A rectangular prism has four faces that are rectangles and two faces that are squares.

A cube has six faces that are squares.

Let's start in two dimensions first:
A square is a rectangle with all sides equal. So all squares are rectangles, but not all rectangles are squares.

Similarly, a cube is a rectangular prism with all sides equal, and therefore all faces of equal area.
A rectangular prism will have 4 faces of equal area and 2 faces of equal area.

So a cube is a rectangular prism, but not all rectangular prisms are cubes.