The Truman Doctrine and the Marshall Plan represented two different ways in which the US was trying to contain communism.
In the Truman Doctrine, the US was promising to support any country that was faced with communist takeover. In other words, this was a doctrine that committed the US to putting out any “fires” that started in any particular country (with “fires” of course being communist insurgencies). The Marshall Plan took a more proactive approach to containment. The Marshall Plan did not wait for a “fire” to break out. Instead, it tried to remove the danger of fire in the first place. It did this by trying to improve the economies of European countries so that communism could not gain a foothold in those countries in the first place.
Thus, the major difference between these two is that the Truman Doctrine was reactive while the Marshall Plan was more proactive.