Understand the Concept of Plagiarism
Random House defines plagiarism as "the unauthorized use or close imitation of the language and thoughts of another author and the representation of them as one's own original work." However, plagiarism in the academic context involves a distinctly different set of concerns than in the journalistic or business contexts. In academia, one's ideas are their most valuable resource. Your teaching assistants and professors probably either did present or are preparing to present an original thesis to obtain their doctorate. Their entire career hinges on their ability to introduce new, novel ideas to the academic community. Needless to say, they are extremely sensitive to any attempt by you, the student, to present the ideas of another as your own, whether intentionally or unintentionally.
Determine Your School's Policy
Focus on the definition of plagiarism or idea theft in your own school's academic code. Salisbury University, a medium-sized university in Salisbury, Maryland, has a typical example: "Plagiarism occurs when a student intentionally or unintentionally deceives or disregards proper scholarly procedures; presents information, ideas or phrasing of another as if they were his/her own; and/or does not give appropriate attribution to the original source. Proper scholarly procedures require that all quoted material be identified by quotation marks or indentation on the page, and the source of information and ideas, if from another, must be identified and be attributed to that source, moreover students are responsible for learning proper scholarly procedures."
How to Avoid Accidental Plagiarism
From the definitions above, the most extreme act of plagiarism would be to simply substitute one's name for that of the original author. Accidental plagiarism is far more common. I recommend the following to ensure that you are not crossing the line. I recommend including a final check into your proofreading process. As you check your work before submitting it, stop after every sentence and ask yourself one question: "Where did I get that idea?" If, for example, you read Pride and Prejudice and thought to yourself, "A major theme in much of Austen's work is the importance of environment and upbringing on the development of young people's character and morality," you would not need to cite any source. It is your idea. If, on the other hand, you got that thought from the Wikipedia page like I just did, you would cite (using whatever citation format your school prefers) F.B. Pinion's A Jane Austen Companion from 1973.
The Bottom Line
The bottom line is that every sentence you submit as a part of any academic writing is impliedly cited to YOU. If the idea is not yours, you must override that implication and give credit to the actual source. Do not get caught up in the concept of direct quotation versus paraphrasing versus complete rephrasing. You will know when you are directly quoting a work. It does not matter where the commas, verbs, subjects and adjectives fall. It does not matter that you used a thesaurus to replace the original writer's choice of the word "rude" with "discourteous". These concerns are simply related to syntax and semantics. Plagiarism prohibits the theft of substance.
Wikipedia and the Internet
The Internet is the most revolutionary development in academia since the printing press. Wikipedia is perhaps the best organized repository of information within the vast reaches of the online community. With all of that information so easily available, students find it far too easy to accidentally repeat ideas and concepts taken from Wikipedia articles without attributing the original source. Worse, because Wikipedia pages are fluid, students can rely on the source given in a particular article even though the statement no longer is supported by the cited footnote. You MUST get in the habit of citing information gathered from the Internet and you MUST make every effort to find an offline source supporting the information you gather. Unfortunately, this may require a trip to the library. In the end, though, you can rest assured that a nasty accusation is not hanging over you.