A wetland develops its own water-oriented ecosystem from being saturated with water either seasonally or all year. The aquatic plant life is what distinguishes it. The water can be fresh, salt, or brackish (somewhere between). They are good places for monsters to hide, if they’re fine with being wet all the time, because humanoid species are often uncomfortable with entering wetlands; we tend to like solid ground.
There are four basic types of wetlands, and most are found in temperate climate zones or the tropics, but they can be in polar areas. Many fish species, including ones we create, use wetlands for nursery grounds, and many animals are typically found in them. This means food and products come from wetlands, including rice, honey, sugar, natural medicines, dyes and textiles.
There are far more lakes than we’re going to draw during world building. Usually we depict the largest ones, which can have mires (discussed next) at various places along their shores. Take a moment to state in your files that there are wetlands in various places on these lakes and how far these are from various settlements. While this is good, it won’t result in nearly as many wetlands as there would be, but the goal is not accuracy per se; it is including these potentially useful land features. I suggest that after building settlements, world builders take a moment to determine which settlements have a wetland nearby, what kind it is, and where it’s located in relation to the community. This will necessitate deciding there’s a smaller lake nearby, the kind not drawn on a continent-sized map.
Bogs and fens are similar types of mires. Both get water from rainfall, but fens also have surface water (water that collects on the ground, contrasted with rain or a spring). The water quality is therefore different, but your readers will not care. Both form at the edges of a lake and can eventually cover the entire surface.
A bog forms in land depressions or old lakes, typically in the mountains in colder temperate climates. They collect deposits of dead plant material (like mosses) and form peat. The water is acidic and sometimes comes entirely from rain partly because the bog is domed-shaped land that is higher than its surroundings. A bog can be many meters deep over a wide area. Evergreen plants can grow there, including trees in drier locations, in which case the bog can appear to blend in with an evergreen forest adjacent to it.
Sometimes carnivorous plants exist in bogs. They survive by eating invertebrates. Or we can have them eat bigger animals and our species. Some large animals like moose and caribou are found in bogs, as are otters and smaller animals. Peat is itself a product that can be used for fuel (heating and cooking).
In Creating Life (The Art of World Building, #1), the possibility of undead plants was discussed. What if a bog contained undead plant material and the remains of either animal or humanoid undead? This could be where the undead emerge from and return to in between terrorizing the living.
Fens are similar to bogs but are covered in grasses and shrubs instead of peat, looking like meadows. They occur along lakes and rivers where seasonal changes in water level occur and are often in the mountains. Fens are located on slopes, depressions, or flat land. Water is supplied from the ground more than rain. A fen may eventually become a bog.
World builders can use fens for inventing plant life that is only found in one. Since fens are typically in the mountains, fetching an herb for a magic spell could place someone in danger from all those nasty ogres and the like that one must bypass to reach the fen. Some of the plants can be more dangerous if carnivorous and willing to consume humanoids. Since a fen looks like grassland, they can be deceptive so that characters enter them before realizing the potential dangers.