A lean-to is term used for two similar, yet different types of building.
It can be a free standing structure of three walls and a sloping roof. The open side is sheltered away from the prevailing winds and rains. Often a rough structure made of logs or unfinished wood and used as a camping shelter. It can also refer to a shed, abutting the wall of another structure, with three walls and a sloping roof.
Lean-to buildings are wood constructions coupled against public or private buildings, the roofs of which have only one gutter. The lean-to building is always provisional; it is an appendix with a turn-key building raised in consequence of new need, or merely tolerated. Today, a great number of public buildings (particularly cathedrals), are surrounded by lean-to buildings raised against their bases, or between their buttresses. These parasitic constructions can eventually ruin monuments, and it is therefore necessary to limit their construction. Sometimes, they cover external staircases, as is the case with the lean-to building built in the 15th century against one of the walls of the large chapter room of the cathedral of Meaux (1). Sometimes, they are built to protect entrances or to establish covered markets around certain large civil buildings.
—Text (poorly) translated from the French language s:Dictionnaire raisonné de l'architecture française du XIe au XVIe siècle - Tome 1, Appentis
Laavus are commonly found in Finnish Lapland near popular fishing rivers. In principle, a laavu is a simplified version of a wilderness hut. Like wilderness huts, laavus are unheated, and may not be reserved beforehand. Unlike wilderness huts, laavus lack doors or windows. A typical laavu is a wooden building, about 10 m² in area and 2 m high, consisting of a roof, floor, and three walls. The fourth wall is left permanently open.
A laavu is intended to only provide a safe place to sleep during fishing trips. Visitors are expected to bring their own sleeping bags, as there are no other sleeping facilities. Most laavus also have a place to hold a campfire in front of them.
A laavu can also be an improvised structure of the same fashion built out of available materials (branches with leaves or pine/fir needles intact or moss or pelts for the covering and sturdier stripped branches or young tree-trunks as the supporting structure) for temporary camp deep in the wilderness, even for a single overnight stay.