We have to assume that "you" sent the customer an invoice, the customer then clicks on a link in the invoice and is redirected to the PayPal Checkout Screen. There the customer has the option to pay with a credit card (as a guest) or login to their PayPal Account to pay you. The money is then deposited into your PayPal Account Balance. You should receive an email notification that you received a payment and there should also be a notice in your Account Transaction History with the transaction details.
