Tree trimming to reduce risk includes clearance pruning, weight reduction pruning, trimming trees for storms, and trimming trees near power lines. The risk you are aiming to reduce steers you to the appropriate method. If you have a mature tree composed of large, heavy limbs that over hang the house, the street, and power lines, then multiple actions may need to be applied.
Clearance pruning is tree trimming to reduce risk by removing limbs to achieve a certain distance away from structures and above gardens, driveways, footpaths, and streets. Driveway clearance can reduce the risk of tree parts falling onto parked vehicles.
Weight reduction pruning is tree trimming to reduce risk by alleviating weight off of heavy limbs. Decreasing the weight on the end of a limb will minimize the chances of breakage. This can be targeted to a single branch, a section of a tree, or the entire tree. The determining factor when assessing risk is the present target.