A Cube has 6 faces, all being squares of the same size.
It has 8 vertices (corners) and 12 edges.
It has 4 'space diagonals', all the same length.
A 'space diagonal' is a straight line joining two opposite vertices which do not belong to the same face. (If they do belong to the same face, it is a 'face diagonal'.)
With this shape it is only necessary to know one of its dimensions, and all the others can be derived from that.
Where (for brevity) it says 'edge', 'space diagonal' and so on, it should, more correctly, be something like 'size of edge' or 'edge-length' etc.