How does a health insurance deductible, copay, coinsurance, and out-of-pocket maximum fit together?

These four terms describe who pays what and when. The deductible is what you pay yourself before the plan starts sharing costs. A copay is a flat fee for a visit or prescription. Coinsurance is your percentage share (say 20%) after the deductible is met. The out-of-pocket maximum is the absolute ceiling on your spending for the year; once you hit it, the plan pays 100% of covered in-network care.

Preventive care is free under the ACA regardless of your deductible. When comparing plans, don't fixate on premium alone. Add the annual premium plus your realistic expected use up to the out-of-pocket max. A low-premium, high-deductible plan can cost more if you use a lot of care, and less if you rarely do.