Today is an exciting day in US healthcare history: For the first time, uninsured US residents can go online to shop for individual health insurance policies and feel confident of a few things: they can easily see information to make meaningful comparisons between plan options; they won’t be rejected or charged an astronomical rate based on their health history; and once they have a policy, they won’t be unpleasantly surprised by an omission of an essential benefit like hospital or maternity care.

In addition, insurance shoppers with incomes between 100% and 400% of the federal poverty level are eligible for subsidies to help defray premium costs. For a one-person household, 100% FPL is $11,490, and 400% is $45,960; for a four-person household, those numbers are $23,550 and $94,200. Check out Families USA’s helpful chart for other amounts and household sizes, or the New York Times’ graphic of sample premium amounts.

What’s starting today is online shopping for individual health insurance plans; people can enroll now, but coverage won’t actually start until January 1. Not every state’s exchange – or marketplace, as they’re now being called – is working at 100% yet, either. Here in DC, for instance, determinations of subsidy amounts will be made offline (rather than immediately online) until some time in November. Many state marketplaces will likely experience glitches and slowdowns, and marketplace teams will be working hard to resolve them in the coming days and weeks. But for thousands of people who are uninsured and don’t know much about the details of Obamacare, they now have a chance to see in concrete terms what it can mean for them.

In about half of the states, people living below the poverty line will be eligible for Medicaid coverage, thanks to the ACA’s expansion of Medicaid to those earning up to 133% FPL. In states that have not accepted this Medicaid expansion, they’ll remain uninsured. The new insurance options exclude some people, however: Only legal US residents can purchase individual coverage through the marketplaces, and only legal residents who’ve been in the US for at least five years are eligible for Medicaid. These exclusions, along with state level-decisions to decline the Medicaid expansion, will have consequences for emergency departments, community health centers, and other parts of the healthcare safety net, as well as for the individuals who’ll remain uninsured. (For more details, we’ve written about the implications of Medicaid expansion decisions here and here and about immigration-status exclusions here and here.)

Many provisions of the Affordable Care Act have already taken effect, and many people have already benefitted from them. Young adults have been able to remain on their parents’ health insurance until age 26; Medicare beneficiaries landing in the “doughnut hole” coverage gap of Part D prescription drug coverage have seen their drug costs decline; insurance plans have been prohibited from placing lifetime limits on coverage; and insurers have been required to spend 80-85% of premium dollars on healthcare services or quality improvement, or else refund the difference to plan members. The Kaiser Family Foundation’s handy implementation timeline summarizes these and many other provisions that are already in force.

A Kaiser Family Foundation poll conducted in mid-September found that the majority of the uninsured didn’t know that they’d be able to shop for coverage online starting October 1. Wonkblog’s Sarah Kliff points out, though, that many state marketplaces are waiting until enrollment opens to do a big advertising push, so we’ll probably see awareness increase in the coming weeks.

This year, enrollment for marketplace plans will run from October 1, 2013 through March 31, 2014, and what’s most important isn’t how many people sign up this month, but how many people are enrolled by the end of the year. The Obama administration and others working to implement the ACA hope that between Medicaid and marketplace plans, we’ll see a big drop in the uninsured US population (currently about 48 million, or 15% of the population). The extent to which currently uninsured or underinsured people – including young, healthy people who might be tempted to forego coverage – enroll will determine the law’s success.