Andy McAfee uses an analogy of an ERP as a factory for business processes. Here are my analogies:

• We are born as originals and die as copies. ERP systems are the other way – they start as copies and die as originals. An ERP system, when it is installed, allows you to configure it by choosing parameters – what kind of budget process, how you define "customer", etc. etc. After having set a few thousand parameters, you can be absolutely certain that you are the only company in the world with that particular SAP or Oracle or whatever configuration. Of course, standardization was what ERP systems were all about when they were introduced in the mid-nineties: The idea that software should be simple again.
• ERP systems are flexible the way cement is flexible. Less true now than it was – cement is ultimately flexible when you pour it, then it hardens into the shape of the hole it was poured into.
• A more advanced version of this is the old joke that SAP (or insert your favorite ERP system) is like a new basic element. Basic elements go through three stages: Fixed, fluid and gas. SAPium (and its cousin Oraclium) start out as a fluid that runs down and fills the holes (basic business process) you want fixed. It then becomes a gas, expanding to fill the whole area (organization) until it has permeated everything, whereupon it becomes a solid that can never be changed again….

Oh well. Less true now than it was, maybe. Or maybe not.