Short-term insurance plans are an alternative option to ACA/Obamacare plans. But, they currently can only be purchased for short periods of time, and typically only cover catastrophic situations. If you have a short-term plan and need medical attention, you will have to pay out of pocket for any circumstances that are not considered catastrophic.
Still, short-term plans can be a good alternative for individuals who just want to be covered in case of emergency for short periods of time, or until they can enroll in an ACA/Obamacare plan.