Is a "Retiree Medical Reimbursement Plan" the same as an HRA?

It seems to me that it isn't, as the "Retiree" arrangement allows participant voluntary after-tax contributions, whereas the HRA's are, as far as I know, employer-funded only.

I don't work with Health Insurance plans at all, and I'm finding the available information pretty fragmented and confusing. Is there a source that anyone knows of that gives a good, solid explanation of what these plans are, and the "rules" in relatively succinct form?

Thanks.