Kinds of plagiarism
The most obvious form of plagiarism is if you take a piece of text and insert it in your essay and pretend you wrote it. How will the teacher know? Trust us, we'll know.
Another form is just as bad - getting someone else to write your paper or finding a paper somewhere and pretending it is yours. This will get you into incredible trouble and again, unless you are the luckiest student on earth and your teacher has no idea what YOUR writing is like, we'll know.
The most common form is (we hope) accidental - while trying to write about someone else's ideas when you are supposed to, you get your ideas and their ideas mixed up. It is important to be clear about where their ideas stop and your ideas start. If this becomes unclear you have probably started to plagiarize.