ERP stand for Enterprise Resource Planning. Wikipedia defines it as "Enterprise resource planning (ERP) is the integrated management of main business processes, often in real time and mediated by software and technology. ERP is usually referred to as a category of business management software—typically a suite of integrated applications—that an organization can use to collect, store, manage and interpret data from many business activities."
Most people have heard of SAP, Oracle or Microsoft Dynamics and think of huge expensive systems that take years to implement and often don't work as expected. While it is true that when a single system is used to conduct all essential functions within an enterprise, there will be compromises. The ERP system will never be as good as the "Best in Breed" tools. however getting the various "Best in Breed" applications to talk to each other can be a nightmare and this is where using an ERP system can be a huge long term advantage.
Implementing an ERP does not have to be a costly and time consuming process. This site is a living example of an implementation done right using free and open-source software.