The <a href="http://en.wikipedia.org/wiki/Robertson%E2%80%93Seymour_theorem">graph minor theorem</a> and the <a href="http://en.wikipedia.org/wiki/Graph_structure_theorem">graph structure theorem</a> are two results which are invoked quite often in combinatorics/graph theory. Much like the classification of finite simple groups they are excellent ways of sweeping hundreds of pages of technical proofs under just a few sentences.