In paintig mode you can modifiy the contents of the actual Layer with the four painting tools.
Paint, Erase, Lighten or Darken with the Left Mouse Button
Move the Layer with the Right Mouse Button
You can change the setting separately for each type of painting tools. There's 9 Brush presets for each tool. You can switch between them from the Brush menu (below) or with the function keys. You can also Copy and Paste the brush settings from here. Brush presets are like several brushes. One is big, one is small and one is smooth. Instead of Loading or changing the settings you can simply swithc between brush settings. These settings are saved so after exit and restart they won't lost.