One large system that brings a company's departments, sales, purchasing, inventory, accounting and HR, onto a shared database.
ERP stands for enterprise resource planning. The idea is that every department works on one system and one set of data: when sales sells, stock goes down and the accounting entry is recorded automatically, with no re-entry. Well-known systems in this field come as ready modules and are configured over months to fit the company, often with the help of a specialist implementation firm.
For medium and large companies this kind of system is essential. A small company, though, often buys one bigger than it needs, pays for modules it never uses and is forced to change its ways to fit the system. The alternative is to build a small system that matches your work as it is, starting with one department and growing. In Fanann you start from your current files with Excel to App, or describe an internal tool you want. Practical tip: list what you actually do every day. If five screens cover it, you need a small tailored system, not a large one. And check accounting and tax requirements with your accountant before relying on any system.