PBX (Private Branch Exchange) is a multi-user telephone system that can be used for many purposes, the most relevant being combining 2 or more PBXs together for redundancy.
Enterprise resource Planning (ERP) typically handles the company's production, logistics, distribution, inventory, shipping, Point of Sale (POS), invoices and accounting in a modular manner.
CRM (customer relationship management) Computer systems to support the management of customer relationships, sales and marketing. Use it to follow up, identify prospects and improve the company's relationship with customers.
POS (point of sale/point of sale terminal) allows the creation and printing of the sales receipt using product references, performs various operations throughout the sales process, as well as changes in inventory. They also generate various reports that help in business management.