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. In 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.
Embezzlement is a form of stealing usually by an employee. The most common form is the taking of cash and goods.