"Wetland" is a term applied to many different kinds of wet areas. Some are underwater part of the year and dry the rest. Some are so acidic that only specially adapted species can survive, others are home to a much wider variety of plant life. Some are brackish, others fresh, others as salty as the sea. Each type has its own characteristics, strongly influenced by the plants that inhabit it.
True bogs are strongly acid, with a pH of 5.0 or below (about as acid as sour milk), and nutrients are much less available than in normal soils. Bog soil is also low in oxygen, one reason why organic matter such as stumps are preserved rather than decaying normally. Only specially adapted plants can tolerate conditions in a bog. These include blueberries, cranberries, labrador tea, pitcher plants and, at the surface, sphagnum moss, the moss that produces peat. Few trees will grow in this type of soil, so most of these plants prefer full sun when grown in gardens.
A marsh is a less extreme environment, less acid, though with a pH still well below average and with more oxygen in the soil. Often, a marsh will be a small lake edged with cattails and perennials such as yellow iris (Iris pseudacorus), red lobelia (Lobelia cardinalis), and marshmallow. There are often floating plants such as water lilies. Trees and shrubs that will grow at the edge of a pond or lake include Oregon ash, Sitka spruce, cottonwoods, willows, birches and aspens.
Wet these edges may be, but the soil is also well-oxygenated because the water passes through quickly, not standing and becoming stagnant. In addition, pH levels are likely to be normal to slightly acid. Here you'll find many of the same species that grow in marshes, plus others, such as vine maple, commonly found in upland areas.
Sometimes there's not much water showing, the soil is just constantly moist. These areas can be depressions in a shady forest, often colonized by ferns, or meadows of grass mixed with rushes and sedges, two kinds of plants often used as indicators of wet soil. There may also be a dense cover of shrubs such as salmonberry, red-stem dogwood and hardhack (Spiraea douglasii).
In many dry areas, spring wetlands often turn into baked summer desert. Even relatively moist areas such as the Pacific Northwest, have dry summers that can force many plants to adapt to drought to survive. Camas, with beautiful blue spikes of flowers in spring, are one such plant growing throughout the west in these conditions. Even skunk cabbage, marked by huge leaves and spathe-like flowers, often has to tolerate summer dry spells.