Plagiarism is to copy/use another person's information without giving them the proper credit.
It is important to cite information because when you don't cite information that you took from another person, it is considered as "stealing" in a sense. Now, if you were to go to a store in real life, stealing would hopefully obviously be illegal. With that said, there is not much difference on the web. If you don't give someone the credit for creating something themselves, then you're stealing their work. Many laws prohibit the use of plagiarism, as does many schools and colleges. Using plagiarism knowingly will undoubtedly result in consequences.