Pruning keeps plants healthy, shaped, and safe, but bad pruning does lasting damage. A few principles cover most situations, and knowing your limits keeps you off a ladder with a chainsaw.
Why and when
Prune to remove dead, diseased, or crossing branches, to shape the plant, and to keep growth away from structures and walkways. Timing varies by plant — many trees and shrubs are best pruned in late winter while dormant, but spring-flowering shrubs are pruned right after they bloom so you don't cut off next year's flowers.
How to cut
Cut just outside the branch collar (the swollen base where the branch meets the trunk) without leaving a stub or cutting into the trunk — this lets the tree heal properly. Use clean, sharp tools, and for larger limbs use a three-cut method so the bark doesn't tear.
Know your limits
Don't 'top' trees (cutting back to stubs) — it damages them and causes weak regrowth. Large limbs, anything near power lines, and big or hazardous trees are jobs for a certified, insured arborist with the right gear.