Using short sentences instead of long ones will improve the flow of the paper and make phrases stand for themselves instead of being academic-sounding gibberish.
The body paragraphs include the analysis and proof; as well as all the aspects of the essay.
It should give an elaborate discussion of the issues mentioned in the introduction.
This also gives a direction for the piece, and the discussed issues in the body of the essay.
The introduction also defines important words that are not explicit to prospective readers.