SaaS stand for Software as a Service, as time moves software application has turned into a service which allows the users to pay for what they use as a service, rather then purchase on one shot. 

POS stands for Point of sale, where a customer picks a product from the shelf and brings it to the counter to be billed. A place where the bill is processed is known as the Point of sales. In terms of software, a application that is designed to scan/option to select the item bought by the customer and able to generate the bill is known as POS.