A bog is a wetland type that accumulates peat, a deposit of dead plant material. The term peat bog in common usage is not entirely redundant, although it would be proper to call these sphagnum bogs if the peat is comprised mostly of acidophilic moss (peat moss or Sphagnum spp.). Lichens are a principal component of peat in the far north. It is typical of bogs that they have no significant inflows or outflows. Moisture is provided by precipitation and for this reason bog waters are acidic.
Bogs are widely distributed in cold, temperate climates, mostly in the northern hemisphere (Boreal). The world's largest wetlands are the bogs of the Western Siberian Lowlands in Russia which cover more than 600,000 square kilometres. Sphagnum bogs were widespread in northern Europe. Ireland was more than 15 per cent bog; Achill Island off Ireland is 87 per cent bog. There are extensive bogs in Canada (called muskegs), Scotland and Finland. There are also bogs in the Falkland Islands.
There exist other terms for what are bogs or peat wetlands similar to bogs. The term moor refers to a flat, boggy area with patches of heath and peat moss (that is, a bog).