Part of the discussion in last week's Democratic debate was over an "individual mandate" for health insurance. Clinton is for it - her health care proposal would require all people to get insurance (like most states require all drivers to be insured) - and Obama is opposed, which is why Clinton can say he wouldn't provide "universal coverage" since some people would choose to go without insurance.
For more, Jonathan Cohn of The New Republic has an even-handed assessment of the issue (and Krugman continues to take Obama to task, as does Ezra Klein). The plan instituted by Governor Romney in Massachusetts also includes an individual mandate.
For a broader look at the candidates' economic policy views, see the series by NY Times' David Leonhardt. Here's his article about Obama, Hillary Clinton and John McCain.