Trimming a tree is cutting back limbs that overhang or interfere with structures or overhead wires. It also includes removal of dead or damaged branches, and branches that rub against other limbs, damaging the cambium layer of both. As in tree reduction, care should be taken to avoid proliferation of suckers at cut points.
Aesthetics of the final result are also a consideration to avoid lopsided or unnatural looking tree shapes. Power companies are often guilty of cutting clearances for overhead wires that render the tree looking lopsided and unnatural.
A properly trimmed tree will look better, be stronger and more resistant to storm damage.