The Health Insurance Marketplace is where individuals and families shop for ACA plans and apply for premium tax credits and cost-sharing reductions. It runs through HealthCare.gov or a state-based exchange.
On-exchange plans are bought through the marketplace (HealthCare.gov or a state exchange) and are the only way to get premium tax credits. Off-exchange plans are bought directly from an insurer or broker; they follow the same ACA rules but are not eligible for subsidies.
For HealthCare.gov, open enrollment generally runs November 1 to January 15. Enroll by December 15 for coverage starting January 1. Some state-run marketplaces use different dates, and a qualifying life event can open a special enrollment period any time of year.