In principle, this means the same thing, we are talking about reduction in size by mechanical action. From a linguistic point of view it is the same thing. From a practical point of view, we will talk about crushing for large materials, and grinding for others. Although to my knowledge I don't believe there is a limit to separate the two.
We rather speak of crushing for the coarse particle size reduction (exp: from the quarry block to granules of a few millimeters), and of grinding for the particle size reduction in the powder phase. These are complementary but different activities.
