Plagiarism is using someone else's words or ideas in your paper without properly indicating the specific source from which you got those words or ideas.
It is expected that you will use other sources to support your paper. No scholar writes in isolation from other scholars. You just have to be careful in how you handle your sources and give credt where credit is due.
From the TEDS / TGS Academic Handbook:
"A student found to break the standard of academic integrity by cheating or plagiarizing will be confronted by the faculty member involved and will be reported to the Dean of Students. This will result in a 'zero' grade for that particular paper or exam, which may result in an 'F' for the course (e.g. depending on the normal weight of that particular assignment, as articulated on the syllabus, the student may fail the course). Such an assignment cannot be 'made-up' or a substitute assignment made. It may also lead to expulsion from the institution, or a revocation of a degree already granted by Trinity."
Intentional plagiarism - To copy a paper from another student, or to copy from some other source (without proper citation), and submit it as your own.
Paraphrasing plagiarism - To closely paraphrase another author's words, that is, to change just a few key words, even if you cite the source for the idea.