According the Merriam-Webster Online Dictionary, to plagiarize is "to steal and pass off (the ideas or words of another) as one's own," or to "use (another's production) without crediting the source." This means that plagiarism includes not only direct word-for-word copying of material and claiming it as original, but also paraphrasing, which is taking the ideas of another and slightly rewording them to appear different than from their original form. Though it is not direct copying, paraphrasing is still the theft of someone else's ideas if you do not give the original author credit.
One of the foundations of the college academic experience lies in the student's synthesis of taught material into something self-created, if not necessarily original. Sure, a student may come up with something that has been discussed before, but it's an essential part of the learning experience that the student's ideas come from her own mental processes and not from the previously published work of another. To plagiarize is to destroy this process, so it is unfair, not only to the author whose ideas are stolen without permission, but also to the student, who is deprived of a vital aspect of learning.
Court cases dating as far back as 1930 show that plagiarism violates copyright law (Nichols v. Universal Pictures Corp., for example), and also trademark and unfair competition law. The latter two, however, rarely apply in the college setting because they refer specifically to commerce-related acts of plagiarism. Also, plagiarism can be considered an act of fraud.
This means that not only is it illegal for a student to plagiarize a paper by taking material from another text, but it also deems illegal the use of purchased term papers not written by the student himself.
In college, professors almost always clearly outline on their syllabi how seriously they take plagiarism and the consequences that could occur if a student is caught copying someone else's work. Punishments range from failing the assignment to failing the course, and students are almost always reported to the university disciplinary committee, where further action may be taken. For example, at the University of Pittsburgh, an act of plagiarism could potentially lead to a student's expulsion from school.
To avoid plagiarism, or a situation that a professor could interpret as plagiarism even if it was an accident, a student should always cite ideas or quotes that are not originally hers. Formats for citation vary by the style the professor prefers (MLA, Chicago Manual of Style, or others) but it always involves citing the author's name and the specific place in the text, such as a page number, that the student has quoted or paraphrased.