What the heck is a health insurance exchange?

And why on Earth is it a major component of all Democratic plans for a government takeover of our health delivery system?
--------------------
A health insurance exchange is an organized marketplace for the purchase of health insurance set up as a governmental or quasi-governmental entity to help insurers comply with consumer protections, compete in cost-efficient ways, and to facilitate the expansion of insurance coverage to more people. Exchanges do not bear risk themselves – they are not insurers. Rather, they would contract with private insurers and possibly offer a public plan option to cover specified populations (such as those obtaining coverage through small employers and those without employer coverage).
Source