What is the difference between the mitotic spindle and microtubules?
Short answer
The spindle is made up of microtubules

Background
From Nature:




  Spindle fibers form a protein structure that divides the genetic
  material in a cell. [...] At the beginning of nuclear division, two
  wheel-shaped protein structures called centrioles position themselves
  at opposite ends of the cell forming cell poles. Long protein fibers
  called microtubules extend from the centrioles in all possible
  directions, forming what is called a spindle. Some of the
  microtubules attach the poles to the chromosomes by connecting to
  protein complexes called kinetochores. Kinetochores are protein
  formations that develop on each chromosome around the centromere,
  which is a region located near the middle of a chromosome. Other
  microtubules bind to the chromosome arms or extend to the opposite end
  of the cell. During the cell division phase called metaphase, the
  microtubules pull the chromosomes back and forth until they align in a
  plane along the equator of the cell, which is called the equatorial
  plane.