Cheating comes in many shapes and forms, but generally it's when a student uses another person's work and passes it off as his own. For example, it's cheating when a student:
Copies answers from another student's test paper
Copies another student's homework
Uses "crib notes," cell phones, or some other method to secretly look at information to answer test questions. Spelling words, definitions and math formulas are good examples
Uses a cell phone or other device to take pictures of tests and exams and sends them to another student, or text messages questions and answers to another student
Pays another student to do his homework
Plagiarism is cheating, too. This may take many forms, but some common examples include when a student:
Buys a pre-written term paper or essay and turns it in as her own
Copies word-for-word another person's opinions or thoughts from a book, magazine, newspaper or online article and passes it on as his own work
Paraphrasing or summarizing someone else's opinions or thoughts and passing it on as her own
As you can see, no matter how cheating is done, the result is the same: The student gets credit for work he didn't do for himself and truly didn't earn.