In case you hadn't noticed, there's a ton of people in the United States who don't have health insurance. Both candidates have their own solution to the problem. The difference between Obama and McCain on healthcare is that Obama's plan actually addresses the fact that millions of Americans, including 10 million children, don't have health insurance. The main problem with this lack of insurance is not only that people don't get care, it's that the care the rest of us get and pay for is way more expensive, because we're paying for those who can't.
McCain's plan is a quintissential conservative approach: free market healthcare. McCain would eliminate the tax incentive companies and businesses currently have to provide their employees with health insruance. It's widely accepted that eliminating the tax break would cause many employers to drop their group insurance plans. In it's place, he would provide a $2500 tax credit for individuals and $5000 tax credit for families to purchase individual insurance. There's one huge problem with these numbers that is evident when you look at this graph that shows the cost of the average health insurance premium in 2007:
An average plan costs $12,000 per year. The numbers don't add up do they?
If you currently have group health insurance through your employer, you may have noticed that they didn't ask you about pre-existing conditions, how old you were or whether or not you smoke crack. If you had to purchase an individual plan in the "free market", people with pre-existing conditions would be unlikely to be eligible for insurance. If they are, the cost would likely be prohibitive except for those with substantial wealth. It's estimated that McCain's plan could result in 20 million more Americans without health insurance coverage. Insurance companies love this plan because they would only have to ensure healthy, rich people, which means huge profit margins that they won't have to pay taxes on thanks to McCain's corporate tax breaks.
So what would Obama do? For one, he would actually address the problem of all those people who currently don't have coverage instead of increasing their numbers. Obama would subsidize care so that people without insurance can get it, which will ultimately result in cost containment. Increasing the number of uninsured, as McCain's plan definitely would, does nothing to contain costs, since the rest of us will end up paying for their care in the end.