We've put a few photos here of different types of penetrating damp. Dont forget it can come from all sorts of places - but common causes include:
Ground levels outside are too high - above the top of the floors internally
Rainwater goods discharging against the wall
Water being drawn through soft and porous stone that appears internally
Cracks or holes in mortar joints
Water being pulled through a very porous stone window sill which bridges from the outside of the wall to the inside. This is a common problem with old stone window sills.
Penetrating damp in Edinburgh
This is a classic example of how penetrating damp can be caused. A stone arched gateway butted up to the house wall, and was not properly flashed to keep water off the join. It ran down the wall, and was trapped in the structure of the gateway - with nowhere else to go, it emerged inside the house, soaking the hallway wall.
Penetrating damp caused by gateway