I don't know a website, so here's a CRASH course.
Managed Care in general refers to utilization control. This can take the form of simple case review by the insurance company or it can be much tighter. For example, requiring pre-approval for certain procedures or hospitalization and capitation (paying the physician a fixed rate per memeber per month). Managed care firms negotiate contracts with physicians and hospitals so they can receive discounts for their members.
A HMO is usually characterized by the dreaded gatekeeper physician and extensive utilization control. The gatekeeper must approve all specialty referrals and hospitalizations. People enrolled in an HMO are only covered to see physicians who are members of their HMO. Not all HMOs use capitation, and not all are closed-staff.
The range of health insurance from regular fee-for-service to closed-staff HMOs is very complex. A popular "mid-range" alternative is the preferred provider organization (PPO). This is basically fee-for-service except the insurance company contracts out a network of physicians to obtain discounts. There is no gatekeeper, and a member can see physicians outside the network if they choose, but it does cost extra to do so.
I hope this basic information helps get you started. The system here is ridiculously complex and people could write for days on this topic.
