KFF released an updated version of its Medicaid primer, outlining the structure of Medicaid, who the program covers, the services it provides, and its costs.  Additionally, the primer explains the effects of the national health care reform law on Medicaid. 

From the introduction:

Over its nearly 45-year history, the Medicaid program has grown increasingly integral to our health care system. Today, it is a primary source of coverage, access, health care financing, and innovation in health care delivery. During the recession, the program has provided a coverage safety-net for millions of Americans, especially children, who would otherwise have joined the uninsured. Under health reform, Medicaid assumes even greater importance as it becomes the national coverage mechanism for low-income people in the new plan for near-universal coverage. With this expanded role for Medicaid on the horizon, basic information about the program is a key resource.

Full report: Medicaid, A Primer: Key Information on Our Nation's Health Coverage Program for Low Income People (PDF | 4.29 MB)

Kaiser Family Foundation. (2010). Medicaid, a primer: key information on our nation's health coverage program for low income people.