What is the Affordable Care Act (ACA) and what does it require?

The Affordable Care Act (ACA, also called Obamacare) is a federal law passed in 2010 that significantly reformed US health insurance. Key provisions: (1) Pre-existing condition protections — insurers in individual and small group markets cannot deny coverage or charge more based on health history; (2) Essential Health Benefits — ACA-compliant plans must cover 10 categories of essential benefits (ambulatory care, emergency services, hospitalization, maternity, mental health, prescription drugs, rehab, lab, preventive, pediatric); (3) Health Insurance Marketplaces (Exchanges) — state and federal websites where individuals can compare and buy plans; (4) Premium Tax Credits — subsidies for households earning 100-400%+ of federal poverty level; (5) Medicaid expansion (state-optional); (6) Individual mandate (federal penalty reduced to $0 in 2019 but still applies in some states); (7) Employer mandate for large employers.