The environmental controls of a starship, space station, or even a colony are used to regulate atmospheric conditions including, but not limited to, the concentration of varying gases, lighting, temperature, humidity, and gravity.
Aboard Federation starships and space stations it is typical for there to be limited access to environmental control within crew quarters in order to meet the varied living requirements of these multi-species structures.
Typically, environmental controls for humidity are set to 37% in standard conditions on Federation starships. (TNG: "Brothers")
Environmental controls have often been used to deploy certain chemicals ship-wide or station-wide. This has been done for both benign purposes, such as the release of retrovirus to cure a ship wide epidemic (TNG: "Genesis") or for more aggressive purposes such as the release of a general anesthetic. (TNG: "Clues", VOY: "Message in a Bottle")
On a larger scale, environmental controls may be integrated into a planet-wide network in order to regulate or moderate global weather. (TNG: "True Q", "Sub Rosa)
The Cardassians prefer warmer locations, thus their environmental controls are set very high. (Star Trek: Deep Space Nine)