While these two terms can be used interchangeably (and often are by us!), strictly speaking pruning refers to removal of dead, damaged, or diseased branches and wood. Trimming refers to cutting back overgrowth to improve not only tree appearance and property accessibility but also long-term safety and tree health.
The word “topping” is sometimes used synonymously with “crown reduction,” but is more extreme and indiscriminate and a harmful practice to most trees. When crown reduction is practiced it must be according to arborist standards with a light hand and the knowledge to know where to cut to help maintain the health of the tree.