The Truman doctrine is the policy of US president Harry Truman in 1947 to counter the growth of Soviet power in Eastern Europe through financial assistance to countries in the region that had not fallen to Communism yet -- more specifically, Greece and Turkey.

The Marshall Plan can be thought of as an extension of the Truman doctrine that emerged later in the same year. It was again a policy of giving financial assistance to non-Communist countries. But this time the assistance was expanded to all of Europe because US Secretary of State George C. Marshall and the US Congress felt that all of Europe was at risk of falling to the Soviets and becoming Communist.

The Truman Doctrine was a bit different from the Marshall Plan in that the Truman Doctrine was about more than just giving money and food to countries in Western Europe.  

The Truman Doctrine basically said that America would provide help (even military help) to any country that was under threat of being taken over by communism.  By contrast, the Marshall Plan provided aid in the form of food and money to countries in Western Europe whether they were being threatened by communism or not.  So the Truman Doctrine was much broader and proactive than the Marshall Plan was.