This is less common, but potentially very useful. And there are others. Other views may differ. As its name implies, action research is intended to achieve both action and research. It is suited to situations where you wish to bring about action in the form of change, and at the same time develop an understanding which informs the change and is an addition to what is known.
Action research is typically cyclic.
The later cycles are used to challenge and refine the results of the earlier cycles. In most of its forms it also tends to be qualitative, and participative. Some would say that neither of these is strictly necessary. Some insist, strongly, on high participation as a necessary feature. Some have defined publication of results as a defining characteristic.