Copyright is a protection that covers published and unpublished literary, scientific and artistic works, whatever the form of expression, provided such works are fixed in a tangible or material form.
According to the Merriam-Webster Online Dictionary, to "plagiarize" means
to steal and pass off (the ideas or words of another) as one's own
to use (another's production) without crediting the source
to commit literary theft
to present as new and original an idea or product derived from an existing source
Fair use is a legal concept that allows the reproduction of copyrighted material for certain purposes without obtaining permission and without paying a fee or royalty. Purposes permitting the application of fair use generally include review, news reporting, teaching, or scholarly research. Fair use is a balancing test to determine whether a use is or is not a fair use. Always keep in mind that you need to apply all four factors.
Factor 1: The Purpose and Character of the Use
Factor 2: The Nature of the Copyrighted Work
Factor 3: The Amount or Substantiality of the Portion Used
Factor 4: The Effect of the Use on the Potential Market for or Value of the Work