Plagiarism
Webster’s Dictionary defines plagiarism as: “to steal and pass off (the ideas or words of another) as one's own: use (another's production) without crediting the source.” This is the primary form of literary theft, that is, the passing off of other’s ideas as your own (Note: This article deals with literary plagiarism, not plagiarism in journalism). The idea of plagiarism is as old as literature, but with the advent of the computer and copy/paste, it has become significantly easier. Samuel Taylor Coleridge is a famous example of a plagiarist; he is believed to have borrowed much of his work in Biographia Literaria from several German philosophers.
Plagiarism is sometimes hard to detect, as even the theft of nothing more than a basic literary idea is considered plagiarism. For example, say I am writing an essay on a topic, maybe Frankenstein. While drifting through a Google search looking for sources for your paper, which deals with how Dr. Frankenstein is the true monster in the work, you come across a separate idea—how Frankenstein is based off the Greek myth of Prometheus. Since you like this idea better than your original one, you change your essay and start again, using only the same (in different words) thesis as the author of the web page you discovered. You copy nothing else, and do your own research. Believe it or not, this is plagiarism. To put it in perspective, it is similar to stealing the “plot idea” of a movie or book and writing a story with that (Such as: “Farm boy must leave his planet and fight the evil Empire and embrace his destiny”, for the original Star Wars movie).
The most common example of plagiarism is the simple copy and pasting of a thought, usually limited to a paragraph or so, but possibly including the entire essay. Using an essay purchased online is also an example of this. Using the previous example, this would be if you had actually copied over a sizable portion of the other author’s work, passing it off as your own. This is the most recognizable form of plagiarism, and therefore the easiest to avoid.
“Rephrasing” an idea is also plagiarism, although it may not appear so at first glance. Recall, however, the definition of plagiarism in the first paragraph: “to steal and pass off (the ideas or words of another) as one's own”. The pure copying of words means nothing; it is the idea that is important.

Works Citied:
"Plagiarism." Wikipedia. 30 Nov 2008 <http://en.wikipedia.org/wiki/Plagiarism#cite_note-1>.
"Plagiarism." Webster's Online Dictionary. 30 Nov 2008 <http://www.merriam-webster.com/dictionary/plagiarizing>.
"Samual Taylor Colridge." Encyclopedia.com. 30 Nov 2008 <http://www.encyclopedia.com/doc/1E1-ColeridgST.html>.