"Plagiarism:
Plagiarism is the act of appropriating or imitating the language, ideas, or thoughts of another and presenting them as one's own or without proper acknowledgment. This includes submitting a paper or part of a paper written by another person as one's own, whether that material was stolen, purchased, or shared freely. It also includes submitting a paper containing insufficient citation or misuse of source material. The unacknowledged inclusion of language, ideas, or thoughts taken from "study guides," such as Cliff's Notes is also a form of plagiarism. (Even when acknowledged, such study guides are too rudimentary to be appropriate secondary sources for a college paper.)" - Drew University, Standards of Academic Integrity 2016

Word-for-Word 
This is easy to spot/understand. If you copy word-for-word from a book, article, person, source, etc, it is an open-and-shut case.

Paraphrasing or changing words slightly
If I were to pitch you a book idea titled A Ballad of Frozen Water and Flames, you'd likely get the reference to the popular book of a fairly similar name. Any time you change the words, but still take the underlying idea from someone else,  you need to credit them.  In other words, you cannot change a quote/passage slightly and claim it was yours.

Borrowing ideas
This is the tricky part. Any time you use the argument or ideas of someone else, you need to give credit. This is true even when you're not directly quoting/citing them.  If you claim "workers should control the means of production" or "tax cuts for the wealthy stimulate economic growth" you need to credit the theorists who gave you the idea.  

So what isn't plagiarism?! 
Building upon the works of others, and using existing ideas to form new ideas, is useful and necessary. The important thing is to let your reader know whose work you're building upon.  If you're still not sure, however, ask your professor or a librarian!