Plagiarism basically can be defined as taking credit for the words and ideas of others, which means you didn't give credit to the actual author.
The obvious acts of plagiarism include directly copying source material into an assignment or paper without proper citations, or using the paper of a fellow student as your own.
MOST plagiarism that occurs
at Rutgers involves students who "cut and paste" sources from the
Internet and take credit for the words. Even using one sentence
improperly can be considered plagiarism! Professors have read many
papers and can usually spot plagiarism pretty easily! In addition,
professors can use computer programs that scan the text of assignments
for possible plagiarism in their detection efforts.
from: Academic Integrity at Rutgers: Easy to Understand Definitions!
Take a few minutes to watch this animated tutorial on What is Plagiarism, created at the Robeson Library, Rutgers, Camden.