(Reuters) - The U.S. Supreme Court is set to hear a second major challenge to President Barack Obama's healthcare law on Wednesday, focusing on the issue of tax subsidies available through insurance exchanges set up under the statute. Here is a look at the exchanges and subsidies. HEALTH INSURANCE EXCHANGES The Affordable Care Act, often called Obamacare, requires people who do not get health insurance through their employer, or the government programs Medicare and Medicaid to buy their own insurance. To make that easier, it created health insurance exchanges, which are centralized online marketplaces that allow consumers to shop among competing insurance plans.