This morning I went to see a flat that had some terible condensation damage.
This is basically caused when the air contains a disproportionate amount of moisture, humidity, and it comes into contact with a surface that is below a certain temperature, dew point.

In this instance, it is a large, north facing wall, with two large windows. The best way to tackle a situation like this is to increase the thermal insulation of the cold surface, increase ventilation of the area, and try to reduce the ratio of water in the air.
It is really important to remove all mould residue, as this will have unwanted effects on respiratory problems and general healthcare.