Simply put, plagiarism is using an idea created by someone else and not giving them credit for it. This can be in the form of words, music, images, etc.
When in doubt, cite it!
This is a partial list of things you should cite:
*Thanks to Donna Zecha, librarian at Hopkinton Middle/High School Library in Contoocook, NH, for this list.
Copyright is the legal right attached to a creative work that prevents others from copying or using it for monetary gain. The copyright owner decides who may use his/her copyrighted work and may request payment for that use.