Plagiarism is theft
Plagiarism is defined as using the ideas or writings of someone else as one's own without giving proper credit.*
In the process of doing a research project you will find supporting material in works written by others. It's okay to borrow and build upon the ideas of other people, but you need to correctly credit the writers and the sources.
If you don't credit the author, you are committing plagiarism.
When in doubt about whether or not to cite, always cite.
*The American Heritage Dictionary of the English Language. (Boston: Houghton Mifflin, 2007)