Constructed wetlands are systems used to treat wastewater, with two main types: free water surface wetlands and subsurface flow wetlands. Free water surface wetlands look like natural wetlands and have water flowing through plants, while subsurface flow wetlands have no standing water and wastewater flows beneath the surface. Constructed wetlands are good for areas with cheap land and limited skilled labor. They have advantages like being cost-effective and environmentally friendly, but also limitations like needing regular maintenance.