There are many reasons why it is important to cite the resources that you consult when researching a paper. The most important of these are:
There are three main ways to integrate another person's work into your own. As you write, it is important to use each method to make your writing interesting and readable.
The three methods are:
Paraphrasing -- To paraphrase something means to put the source material into your own words. Typically, the paraphrase will be shorter than the source material.
Summarizing -- When you need to use long passages, chapters, etc. of source material, you can rewrite the main ideas in much shorter form. However, the ideas belong to the source material so they still need to be cited.