It is important to cite sources you use for many reasons:
What is plagiarism?
It is the idea of taking someone's work (words, ideas, images, etc.) and representing them as your own without giving appropriate credit to the source.
Is This Plagiarism?
I didn't realize I copied. - Yes this is plagiarism! Even if it was an accident or you didn't realize it.
Putting someone else's ideas into your own word and do not provide credit to them by citing sources - Yes this is plagiarism!
Copying images, charts, diagrams, data, etc. and not citing the source - Yes this is plagiarism!
Using a speech, conversation or idea that hasn't been published yet and passing it off as your own. - Yes this is plagiarism!
What if there is no author information? Can I copy without citing my source? - No, that still needs to be cited!
For more information, try Purdue Online Writing Lab’s resources for avoiding plagiarism.