Much more information is available at: http://www.cmmifaq.info.

Summary here:

CMMI stands for "Capability Maturity Model Integration".  It's the integration of several other CMMs (Capability Maturity Models).  By integrating these other CMMs, it also becomes an integration of the processes and practices within the model in ways that previous incarnations of the model(s) didn't achieve.  The CMMI is a framework for business process improvement.  It is NOT an engineering development standard.  Please take a moment to re-read and reflect on that before continuing. 

The business processes of developing engineered products are the focus of CMMI.  This helps understand why it is most widely applied in software and systems engineering organizations. But, where it is applied is a significantly distinct matter from being anything even remotely akin to a standard or certification mechanism for the engineering or technology required to develop products.  If an organization chose to do so, CMMI could be applied in the construction or even media production industries.  (Exactly, how would be an *entirely* different discussion!)

Before we get too off-track... CMMI is meant to help engineering development organizations improve on their capability to consistently and predictably deliver the products their customers want, when they want them and at a price they're willing to pay.  From a purely inwardly-facing perspective, CMMI helps companies get from estimates to actuals in the black. 

Without some insight into and control over their internal business processes, how else can a company know how well they're doing before it's too late to do anything about it?  And if/when they wait until the end of a project to see how close/far they were to their promises/expectations, without some idea of what their processes are and how they work, how else could a company ever make whatever changes or improvements they'd want/need to make in order to do better next time? 

CMMI provides the models from which to pursue these sorts of insights and activities for improvement.  It's a place to start, not a final destination.  CMMI can't tell an organization what is or isn't important to them.  CMMI, however, can provide a path for an organization to achieve its performance goals.

At the risk of seeming self-serving, we further address the question of what CMMI is in the following two writings:

"What is CMMI & Why Should You Care?", http://www.entinex.com/WhatIsCMMI_page1.cfm, and

"Keys to Enabling CMMI", http://www.entinex.com/agilecmmi/2006/08/keys-to-enabling-cmmi.html.

Also, CapabilityMaturityModelIntegration is a more comprehensive page for the same subject and is more frequently maintained. This page will remain available to aide those who may happen to search on the title or contents, but all further additions to the material will be made there, not here.
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Contributed by, HillelGlazer.