These occur when you receive an email from what appears to be a legitimate business offering you employment. As part of your "job" duties, you are sent a check from the company. Your employer asks you to deposit the check in your bank account. You are to keep some of the money for your pay and then distribute the remainder as instructed by your employer.
The scam is that the check or money order sent to you is fraudulent. When your bank rejects the funds from your "employer" you will have overdrawn your account and owe your bank fees and the funds deposited.