Do I have to get insurance? What is the "individual mandate"?

Do I have to get insurance?  What is the "individual mandate"?

Individuals have until March 31, 2014 to enroll in a Qualified Health Plan through the Marketplace for 2014. To have Marketplace insurance coverage that begins in January 2014, individuals must enroll by December 23, 2013.

Who is exempted from the individual mandate?