Affordable Care Act has both open enrollment and special enrollment period for getting health insurance. During open enrollment, you can sign up for health care coverage if you meet the basic requirements.
The current Affordable Care Act open enrollment is from November 15, 2014 to February 15, 2015. It is only during this time, you can can enroll in a Qualified Health Plan in the marketplace. In general, you can only receive government subsidies (tax credits) for policies that are purchased through the marketplace exchange.
Outside of the open enrollment period is called Special Enrollment Period. (Anytime after February 15, 2015 to the next Open Enrollment period).
You only qualify for special enrollment if experience certain Qualifying Life Events. Please note, you can apply for Medicaid or CHIP (Children Health Insurance Program) at any time of the year.