What is an ERP system?
An ERP system is a software package that combines various elements to allow a business to manage its processes in one system rather than have many systems or documents to work from.
SAP Business One is an integrated solution and when you buy the product you get the complete system. Your licence determines what you get access to. This is a great way of building a system over time, so you can add functionality when you need it rather than have all areas forced on you when they are not required.