Plagiarism is using someone else’s ideas or words as one's own without giving them proper credit. Plagiarism can range from unintentional (forgetting to include a source in a bibliography) to intentional (buying a paper online, using another writer’s ideas as your own to make your work sound smarter). Beginning writers AND expert writers can all plagiarize.
It is important to recognize that standards for citing sources varies, but in all situations we must attribute other people’s words and ideas to their appropriate source.