Ask ten people and you'll get ten definitions, but in practice the difference comes down to purpose. Trimming is about the space around the tree. Pruning is about the tree itself.
Trimming is what most homeowners call about first: limbs rubbing the roof, branches hanging low over the driveway, a canopy blocking the porch light. The goal is clearance and a clean shape, and the work is usually done on the outer parts of the tree.
Pruning goes deeper into the structure. It means removing dead or diseased wood, taking out branches that cross and rub against each other, and on young trees, choosing one strong central leader so the tree doesn't split in two later. Good pruning is selective. It removes the right branches rather than a lot of them.
Both jobs share the same rules for cutting. Cuts are made just outside the branch collar, the slightly swollen ring where a branch meets the trunk, so the tree can seal the wound. Leaving long stubs or cutting flush against the trunk both slow healing and invite decay.
Many visits end up being a mix of both: clearance where the tree meets the house, structural work where the tree needs it. When we walk your yard we'll tell you which is which, and if a tree only needs a light touch, that's what we'll recommend.