Basically, enterprise resource software is intended to be total software solution packages for running large businesses.
Large businesses do a lot of different things, and therefore have a lot of different software needs.
They want to be able to buy one package that does everything from keep track of sales orders, to payroll, to manage inventory, to spit out their end of fiscal year tax return and ERP is trying to fill that need. It is closely followed by Peoplesoft, Baan and Oracle. However, it is impossible for ER P software's original programmers to design software that covers the needs of every single large business. Every business is different. They manufacture different products.