Let's start with a basic definition: Plagiarism means taking words or ideas from a source without documenting them.

Words: any words (even one or two) taken from a source and included in your own work need to be in quotation marks and documented. There may exceptions to this (common knowledge), but when in doubt, use quotation marks and document or check with your instructor.

Ideas: when you get an idea from source, be it from an article, lecture, person, etc., it must be documented – even if you put it in your own words (and if you use the same words, they must be in quotation marks).

Source: the person, book, article from which you obtained information.

Documenting: following a particular documentation/citation style such as MLA or APA to let the reader know where you acquired your information.