Most cracks on plaster are due to age. Over the years, plaster can separate from its lath and show up as cracks on the surface - this is cosmetic damage and can be fixed. But if plaster cracks are diagonal or jagged and measure more than 6 millimetres in width, this can be a sign of a more severe problem. These can be signs of a shifted foundation or deteriorated or collapsed wood beams due to termites; you may hire pest control to inspect.