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:
Submitting someone else's work as your own.
Copying from someone else's work without giving the original author credit.
Buying a paper from someone.
Copying and pasting text, graphics, video, music from Internet sources.
Many instructors will automatically fail students who are found guilty of plagiarizing.
Here are some websites that are good resources about plagiarism.