What causes water stains on my ceiling, and who fixes them?

Ceiling stains usually mean water from above: a roof leak, a plumbing leak from a bathroom, or condensation/HVAC. Note if it grows after rain (roof) or after using an upstairs fixture (plumbing). Fix the source first, then seal and repaint the stain (a stain-blocking primer stops it bleeding through). Call a roofer or plumber depending on the pattern.