Harry Truman is generally seen as a strong president. He dealt with some very important issues in foreign policy fairly effectively. These included the conclusion of WWII (and the decision to use the atomic bomb), the start of the Cold War (with such things as the Marshall Plan) and the Korean War. While he did not do everything perfectly, he managed to keep the US strong and give it direction as the Cold War began.
Domestically, Truman is not identified with any dramatic changes. He had no huge domestic programs to his name. However, he did preside over a successful transition from a wartime to a peacetime economy.