The Coal Generator is a generator that allows early power generation. It burns coal to produce electricity. There is a short warm-up period, after which it will gradually increase power output up to 10 kW (or in Galacticraft 3, 120 gJ/s). You can connect it to any machine that requires electricity using Aluminum Wire.
The Coal Generator accepts coal or Blocks of Coal. A Block of Coal lasts 10 times longer than regular coal. (In versions of Galacticraft prior to 188.8.131.526, it lasted 9 times longer.)
Pro Tip: The Coal Generator works more efficiently - makes more electricity for the same amount of coal - if is kept running continuously so that the hull heat and generation per second remains high. But, its internal electricity store will fill up quickly - 1 coal is enough to fill it if you are not using the electricity. So, it's definitely a good plan to connect it to an Energy Storage Module as soon as you have the materials to make one.