Marsh plants, sometimes called poolside plants or bog plants, are those species which can grow in permanently wet soil, but which cannot tolerate flooded soil or having their crown or foliage underwater all year round. Many plants sold as pond plants are in fact marsh plants.
Marsh plants can be planted around the edges of the pond, where the soil is wet but not waterlogged. All marsh plants will also grow perfectly happily in a standard flower bed as a 'normal' garden plant - they will obviously not tolerate very dry soils or drought, but they do not actually require wet soil in order to grow well. Like marginal plants, marsh plants generally prefer a position that is sunny to partial shade. However, as a general rule they are slightly more tolerant of shade than marginal plants are (because marsh ecosystems often contain trees, such as willow and alder, whereas pond ecosystems do not).
There is an extremely wide range of marsh plants (far wider than true pond plants). Here at Water Garden Plants we focus on a few native marsh plants which are difficult to find elsewhere, or that we simply like ourselves. If you have a large marsh area to plant, it might be worth looking at the "moisture-loving" sections of standard nurseries as well.