In the Internet age, word-for-word copying generally consists of copying something from the Internet and pasting it into your paper. This can be as much as an entire paper or as little as one sentence. If you've copied something word-for-word without referencing a source, you've plagiarized. If you need to use a source's exact words, use quotation marks and cite the source using your preferred citation style.
Say you have to write an argument for class. In your research you come across another paper making the same argument you want to make. Instead of using that paper as a source, you take their ideas and present them as your own. Even if you rewrote everything in your own words, you are still guilty of plagiarism because you have presented someone else's ideas as your own. If you're using another person's ideas, it's important to give him credit by citing him as a source.
In an academic environment, self-plagiarism typically occurs when a student turns in the same paper for more than one class without the instructor's permission. This is considered dishonest because you are not doing the work required for the course. In many cases, the instructor may allow the use of a paper written in another course as long as significant revision will occur, but it is important to clear your ideas with your instructor first in order to avoid being accused of self-plagiarism.
The most important thing to do when writing an academic paper is to cite your sources as soon as you've added their words or ideas into your paper. It is also important to keep good records when taking notes from research. It is not uncommon for students to unintentionally plagiarize because they didn't realize they had recorded verbatim from their source in their notes. Finally, it is important for a student to know how to quote, summarize and paraphrase properly.
The consequences of plagiarism vary based on the severity of the incident. If your instructor believes you unintentionally plagiarized, you may get away with nothing more than a warning. However, in most cases it will result in the student earning a failing grade for the plagiarized paper, and depending on your college's practices, you could receive a failing grade for the course. In instances of repeated plagiarism, a student could face expulsion from the institution.