There's always some moisture in the air, even if you can't see it. If the air gets colder, it can't hold all the moisture and tiny droplets of water appear. This is condensation. You notice it when you see your breath on a cold day, or when the mirror mists over when you have a bath or shower.
Condensation is mostly a problem in cold weather. It appears on cold surfaces and in places where there is little air movement - in corners, on or near windows, in or behind wardrobes and cupboards. It is made worse by inadequate ventilation, insulation and heating.
When this water vapour comes into contact with colder surfaces, such as a window or wall, it can soak into wallpaper or paintwork or even plasterwork.
If this continues over time, it can cause black mould to grow.