A Point of Sale (POS) is the place were a purchase and payment is made, i.e. the cash desk. POS systems often go beyond pure payment processing and include retail management functionalities like customer relationship management (CRM), inventory management and financial reporting.
POS is not to be confused with Point of Purchase (POP) which is the location where the buying desicion is made by the customer. This can be in a broad scope the entire shop or in a narrow view the location the customer takes the product out of the shelf into the cart.