Unless you know the difference, it's common to use Pruning and Trimming interchangeably but there is a key difference between the two.
Pruning is removing dead, infected, and/or loose branches or stems from a tree or plant. Trimming is cutting living stems or branches back to the desired length. Very crudely put, Pruning, the part being removed tree or plant is dead. Trimming, the tree is overall healthy. Pruning is often for the plant, tree, or property's safety. Trimming is more for visual purposes.