In a nutshell, the scam occurs this way: the scammer asks for your business services, whatever they might be, and sends you a check that’s much bigger than the amount you asked.
Then, the crook acknowledges his mistake but gives you “green light” to deposit the whole amount in your account, if you send him the difference. You do this and the original check bounces a few days later, leaving you poorer by the amount you sent him.
For exact examples with different twists, check the Humanitarian jobs scam, Mystery shopper scam, and Tutor for hire scam (all under Employment category), as well as the Roommate moving to town scam (House/ Properties) and the Vacation rental by renter scam (Travel).
How to avoid: never deposit a check with an amount bigger than you expected. Ask the scammer to send you a new check. He won’t and you will lose the money for the services, but that’s better than losing large amounts of money from your bank account.