Topping is the practice of pruning trees by indiscriminately cutting branches off the top or "crown" of the tree with no regard for tree health or structural consideration. This damages the tree.
An educated Arborist does not top trees. Instead, we prune them, which encourages growth, improves tree health, repairs damage and adds to the aesthetic appeal of the tree. The picture below is an example of two branches previously topped. Topping encouraged suckers to sprout out of the top of the two center branches. Suckers grow quickly, weaken the tree, and look bad.