An ERP is a combination of Business Management and Technology. The
functionality of different systems is integrated to form an ERP where the
individual applications share information and assist with the core
functionality of an enterprise as a whole. They put across the information
depicting the key performance index (KPI) of an organization. They help in
planning, design and decision making. The individual system could be
in-house built or third party supplier systems enabling sharing of required
data between the organization, vendors, customers and the suppliers.
Each organization has its own challenges to meet the growing customer
demands, market competitors and to deliver on time. On time delivery can be
achieved only if there is a proper plan in place. For example let us
consider a food industry; nobody buys the ingredients for the product years
in advance. They would definitely get wasted. At the same time they need to
anticipate the demand for the product for the coming cycle and should have
a margin to expand their sales. So for a food industry the average time
required to dispose an order could be one week, one month etc based on the
product. Now let’s consider a software product. I wonder where we would
have landed if we could develop new products over a week. Definitely the
turnaround time here is more and the plan is for a longer period. Likewise
they are several other factors that have to be taken into consideration; a
product may experience high sales during summer and low during winter,
there could be an internal dependency (Sales of one product, could have an
impact on the other) etc. An ERP model is a blending of Business logic's
with technology by integrating systems that share data for analysis to
arrive at appropriate conclusions. There may not be an exact match in the
predictions however a well implemented ERP gives the best fit to growing
business needs and reduce wastage.

ERP technology is divided into the below modules based on the business
purpose they serve
  FRM – Financial Resource Management
  MRP – Manufacturing Resource Planning
  CRM – Customer Relationship Management
  HRM – Human Resource Management
  SCM – Supply Chain Management

The diagram shows a rough sketch of MRP module as I have been considering
product manufacturing as an example in my Blog.
The referential system here holds the basic information (Product Names,
Suppliers, and their Contact Information etc) All other systems use this as
a base for fetching the basic information.  The planner holds the bill of
material or the recipe to build a product. The information provided by the
planner is substituted by the actual values when a customer places an
order, the forecast systems help to predict the upcoming sales/market
conditions; there are systems through which the third party suppliers and
vendors access information, place orders, assist with sales, distribution
and marketing. The model also includes systems which generate invoices,
send bills to customers and for making payments. The other modules (FRM,
SCM, CRM, HRM) also hold an integrated group of systems which assist with
the proper functioning of an enterprise.