What is plagiarism? Using another person's ideas, expressions or work and passing them off as your own is intellectual theft, or plagiarism.
After I copy/paste, I always change the words around. That's not plagiarism, is it? Yes, it is. When you copy/paste, you MUST use quotation marks and cite your source. If you are not using quotation marks, then you can't copy/paste--you need to put the ideas into your own words and cite the source(s) of the ideas.
How will anybody ever know? Plagiarism can be easy to detect, usually by a change in writing style or uncharacteristic sophistication of ideas. It can also be detected using plagiarism-detecting software and services, or merely by performing a web search.
What is the big deal? Plagiarism carries serious consequences. In high school it will get you, at a minimum, a zero on the assignment. Many colleges and universities consider it sufficient reason to expel students. Plagiarism can seriously damage careers and reputations:
Judge could be removed from bench if he plagiarized seven years ago Tampa Bay Online, Tampa, Florida
Plagiarism gets you fired KESQ News Channel 3, Palm Springs, California
Isn't the information on web pages free to be used? It is free for you to read. If you use the ideas/information in your own work, you must cite the source(s) or you have plagiarized. If you copy/paste, you must use quotes as well as cite the source. Additionally, all information published on the web is protected by copyright, so you can be in violation of federal law when you use copyrighted material that you don't have permission to use.
Citing Resources: MLA University of North Carolina at Chapel Hill Libraries
Information Ethics Tutorial, University of North Carolina at Chapel Hill Libraries
Copyright/Plagiarism Tutorial, Scholarly Communication Center of the NCSU Libraries
How to Avoid Plagiarism, University of Michigan Shapiro Undergraduate Library