The main parts of the Constitution of the United States are the various articles of that document. There are seven articles, the first three of which the most important.
The first three articles of the Constitution lay out the powers of the various branches of government and set up the methods for selecting members of those branches. Article I deals with the legislative branch, Article II with the executive, and Article III with the judicial.
Article 4 through 7 are more miscellaneous. They do things like mandating that criminals (or slaves) who flee a state must be returned to that state (or to their owners). They set up the manner in which the Constitution can be amended and they establish that the Constitution will be the supreme law of the land.
Overall, though, the first three articles of the Constitution are the most important because they set up our system of government. In this set up is contained some of the most important principles of the Constitution, the idea of separation of powers, the idea of checks and balances, and the idea of limited government.