Skimming is a generic term referring to taking a little bit off the top. In dairy, it refers to the cream at the top of the milk pail. With painting, it refers to a very thin coat of paint to identify imperfections with the wallboard. In business, it means taking a little bit of the revenue without recording the revenue to the books of record. It is very common in your cash based operations.
Cash receipts refer to accounting for the revenues and payments received from customers for accounts receivable. There is a difference between cash receipts and revenue. Revenue is a function of sales, receipts is a function of actual cash payments received by the company (sales, customer payments on accounts, refunds paid to the company etc.).