The true definition of HMO (if there even is one) varies widely state to state, plan to plan. Some are entities where the insurance co and the clinics/hospitals themselves are owned by the same company. Others are separate.
What all HMOs generally have in common are the administrative requirements for the members who seek specialty care. Basically, in order for an HMO to see a specialist, they must first see their primary care physician (PCP) and obtain an administrative referral. An “administrative referral” is where the PCP formally notifies the insurance company of the referral. The referral is to a specific specialist for a specific type of care and a specific number of visits. Should that specialist say that you need to see a different specialist, you need to go back to your PCP to obtain another administrative referral.
Sometimes there are benefits out-of-network - where a member would self-refer to a specialist of their choosing. Sometimes there is no coverage except for (generally) emergency care. Some states offer exceptions to this rule. In MN, for example, women may self-refer to an OB/GYN within a larger type of network.
The idea back when HMOs became popular in the late 80s and early 90s was that plans would save money by restricting self-referrals to specialists (who cost more) when the presenting problem wasn’t complicated enough to warrant a specialist. In addition, many plans administering HMO type plans included financial incentives for PCPs who did not refer beyond a certain patient per month ratio. Or, PCPs were penalized financially when they did refer beyond a certain patient per month ratio.
Problems arose because of high administrative costs for the health plans. They now had to process and review all of these referrals (which means more staff to handle all the extra paperwork). There was a lot of claims adjustments because maybe the PCP didn’t get the referral in the system quickly enough and the claim for the specialist came in before the referral. Or a member would self-refer, the claim would deny or pay at a reduced rate, then they would badger their PCP until they provided a post-service referral. It got somewhat messy/costly on the administrative side.
HMOs also received negative press from both doctors and patients because the insurance companies were perceived to be managing folk’s care instead of the doctors - and in some cases this was true to a degree - in order to save money.
So - no one was really saving money with HMOs, doctors didn’t like them, patients didn’t like them and they just fell out of vogue. They are still available in many areas, but aren’t as ubiquitous as they once were.
HMO type plans can tend to cost the member less than a sort of open network type of plan with similar coverage. This can really vary.
If you’re young, relatively healthy, and don’t have a history of more complex health issues - plus you don’t mind locking in on one primary care doctor to manage all your health care needs - plus the additional red tape involved should you need to see a specialist, there can be value in these plans.