The term "bog" is widely used to describe a number of wet habitats, but ecologists use the term more narrowly to describe a very specific plant community type.
Bogs (in the narrow sense) occur only on deep saturated peat. The surface of a bog is isolated from ground water and from water that flows from mineral soil, so it's only source of water is rainwater. This makes bog water very low in mineral nutrients and very acidic so only very specialized plant species can survive these conditions.
Bogs are typically dominated by hummocks of sphagnum moss, with scattered shrubs such as leatherleaf (Chamaedaphne calycylata) and Labrador tea (Ledum groenlandicum). There may be a few stunted trees, such as black spruce (Picea mariana) or tamarack (Larix laricina).
Because bog waters have very few nutrients to support plant growth, carnivorous plants are common in bogs, such as sundews and pitcher plants that capture and digest insects.