First of all, you can read the Declaration of Independence for yourself to see exactly what it has to say. There is a link to the Declaration below this answer. If you examine the Declaration carefully, you will find that there are five sections in the document. Each has a slightly different purpose.
The first section is generally called the Preamble. This paragraph (it is actually just one very long sentence) gives the purpose of the Declaration. It says that the colonies are planning to separate themselves from Britain and that they owe it to the world to explain why they are going to do that.
The second section lays out the political philosophy behind the Americans’ desire for independence. It says that government should only exist in order to preserve its citizens’ God-given rights. It says that governments are only legitimate if they exist by the consent of their people. Finally, it says that if a government does not exist by the consent of the people and/or does not protect their rights, the people have the right to break away from that government. This is the most famous section of the Declaration.
The third section is something of a propaganda section. This section lays out the colonies’ grievances with the British government, blaming them all on the King himself. It claims that the King has done all sorts of things to abuse the rights of the colonists. This section is meant to show that the British government has not been protecting the rights of its people.
The fourth section is meant to make the colonists look good. It asserts that the colonists have done everything they could to try to patch things up with the British. Finally, the fifth section (another very long sentence) sums up and formally declares that the colonies are declaring independence for the reasons already given in the rest of the document.