The Marshall Plan, NATO, and the Truman Doctrine all kept the USSR from expanding their power. What was the difference between the Marshall Plan and the other two in terms of how they did this?
The major difference between these factors is that the Marshall Plan did not rein the Soviet Union in militarily while the other two did.
One way to keep Soviet power from expanding was to contain it through the threat of military force. The Truman Doctrine implied that such force would be used. The creation of NATO made it clear that military force would be used if the Soviets tried to expand. This was a very straightforward way of trying to contain communism.
However, the Marshall Plan (and, to some extent, the Truman Doctrine) also helped to contain communism in a more subtle way. Communism could potentially have spread through non-military means. That is, it could have spread if Europeans had felt that it was in their best interests to become communist. The Marshall Plan helped to prevent this. It helped to ensure that the countries of Western Europe could return to relative prosperity. People in relatively prosperous countries are much less likely to become communist than those who live in impoverished countries.
Thus, the Marshall Plan contained Soviet power economically while the other two were more focused on military containment of the Soviets.