Very roughly speaking, each school of thought differs by the set of economic phenomena they wish to explain, the economic methodology used, and the assumptions (either explicit or implicit) used in order to explain those economic phenomena.
The most detailed website I've seen on economic schools of thought is the The History of Economic Thought website. They've got dozens of pages on such schools of thought as the classical school, the monetarist school, the Keynesian school, and Austrian economics, just to name a few.
Be sure to visit the Macroeconomic Schools of Thought page on this site, which contains links to five major schools of thought in macroeconomics: Neoclassical, Keynesian, Austrian, Marxist, and Open Economy Macroeconomics.
