Think of a sphere as an orange. To cut a segment you position your knife at the centre and make a cut to the edge and take your knife vertically down along the diameter to the cutting surface. Then you make a similar cut at an angle to the first. You then remove the piece you cut out. That's a segment. (Cadbury's chocolate oranges are divided into segments.) Take the same orange and your knife and, starting anywhere on the circumference cut straight across to another point on the circumference like a guillotine. The piece you cut off is a sector. If you cut an orange exactly in half the segment and sector have the same shape.