One is a manual type that you must light, and the other is an automatic pilot ignition type that lights itself. Turn off the electrical switch that controls the boiler . If you have a modern pilot light that's gone out, you should contact a professional to relight it. But, if you have an older gas-boiler pilot light, you can relight it with. Once the gas has had time to dissipate, turn the knob to "Pilot." Then, hold down the "Reset" button and bring the flame of a long lighter close to the pilot light.
If yours was made pre it's likely to have a pilot light instead of a reset function, so check that the pilot light is on. If it's not, you may be able to relight it.