Oxford Dictionary of Difficult Words states that, “plagiarism is taking the words or ideas of someone else and pass it off as one’s own”.
How to Avoid Plagiarism
1. Give yourself ample time for profound research.
2. Take careful notes of sources used, so that you do not forget them by time you need to include them in your bibliography.
3. Allow yourself sufficient time for revising your paper.
4. Check your Writing professors' syllabuses. Most of them include enough information on research and writing and how to paraphrase and quote.
To legally and ethically use information created by somebody else, you can use one of the following techniques:
1. Quote
Quoting someone else's words is fine, as long as you use quotation marks to indicate the exact words of the original and cite the source properly.
2. Paraphrase
Using your own words to describe someone else's idea. You have to be careful not to change the meaning of the original idea. Credit should be given in this case as well.
3. Summarize
Summarizing is similar to paraphrasing, and the same requirements apply. The difference is in the ammount of information that is condensed. In summarizing it is bigger.