There are two types of plagiarism, intentional and unintentional.
Intentional plagiarism occurs when a student knows that he or she is passing off someone else's words or ideas as their own by purchasing a paper, turning in a friend's paper as their own or copying and pasting from the internet or other source and not giving credit to the source.
Unintentional plagiarism is the most common type of plagiarism and occurs when students use the words or ideas of others but fail to quote or give credit, usually because they do not know how. Examples include omitting a citation or citing inaccurately, paraphrasing by only changing a few words without changing the sentence structure of the original text.
Even if you did not intend to cheat, it's still plagiarism! The punishment can still be the same whether or not you meant to cheat or not.
A flowchart for plagiarism for journalism, but can easily translate to academics as well.
Source, including PDF of flowchart: http://www.poynter.org/latest-news/mediawire/269273/is-it-original-an-editors-guide-to-identifying-plagiarism/
All knowledge is built from previous knowledge.
When you put your ideas on paper, your instructors want to distinguish between the building block ideas borrowed from other people and your own perspective.
So in the majority of research assignments you'll get in college, your instructors will ask you to read something and then write a paper in which you analyze one or more aspects of what you have read.
Essentially, your instructors are asking you to do three things:
Source: Plagiarism Handout from the University of North Carolina, Chapel Hill.
When you cite a source, you're using an expert's ideas as proof or evidence of a new idea that you are trying to communicate to your instructor.