Plagiarism is when someone uses someone else's ideas, or work and doesn't acknowledge where it came from. In essence, they try to pass those ideas off as their own (OED). One thing to remember is that even if someone else's words are being used unintentionally it is still plagiarism.
The first thing to do to ensure that you don't plagiarize (even accidentally) is to cite every idea that isn't yours.
It is ok to take people's ideas and reword them, but you still have to cite the original person.
A few tips from the Harvard College Writing Program to help ensure that you don't plagiarize:
If you would like some additional resources related to avoiding plagiarism or the best practices for quoting and paraphrasing, check these out:
The Colleges take academic integrity and plagiarism very seriously. These policies are explained in great detail in the:
The Writing Fellows at the CTL and the Writing Colleague assigned to your class are all happy to help you appropriately quote, paraphrase, and use sources in your papers.