Defining and Avoiding Plagiarism:
The WPA Statement on Best Practices

Plagiarism has always concerned teachers and administrators, who want their students' work to represent their own efforts and to reflect the outcomes of their learning. However, with the advent of the Internet and easy access to almost limitless written material on every conceivable topic, suspicion of student plagiarism has begun to affect teachers at all levels, at times diverting them from the work of developing students' writing, reading, and critical thinking abilities...

See the full text here.