Plagiarism is when you take credit for ideas or words that aren't your own. It is a form of stealing. A person who plagiarizes another's work is committing theft of intellectual property.
Some examples of plagiarism are:
Submiting someone else's work as your own.
Copying from someone else's work without giving the orginal author credit.
Buying a paper from someone.
Copying and paste text, graphics, video, music from Internet sources.
Many instructors will automatically fail students who are found guilty of plagiarizing.
Here are a couple of websites that are good resources about plagiarism.