How do I know what plagiarism is?

A few weeks ago, my teacher igned us an essay comparing Hamlet and Holden Caulfield. I did the whole essay without any outside sources apart from copies of the play and novel. Today, my teacher told us that there have been several cases of plagiarism on the essay. The students who come forward by tomorrow will receive a 0. If the students do not come forward, they will receive a 0 and be disciplined through the office. What do you think my teacher meant by "plagiarism" My usual paranoid self just spent a couple of hours searching "Hamlet vs. Holden" on google. One of the first things that comes up is an essay written on the topic and posted on a message board. In the essay, the author quotes a line from Hamlet that I quoted in my essay. He or she also compares the two characters as both being cynical about the world. I did not plagiarize, and if we used turnitin.com, it would show up as 0% plagiarized (no words match) Could I be accused of stealing ideas?