Ok.. few basics.
- Pallete = a collection of colors. GIMP comes with quite a few, and you can create you own. If you use the same x colors all the time, it's easier to create a pallete and save than to try to find the colors by the hex value.
- Layers = a stack. Things on top (higher up in the layer order) can cover things below it.
- Channel = a special type of layer made of a black and white image. Channels are created by creating a selection, and then using the Selection->To Channel menu item. Channels are one way to quickly create a mask.
- Layer Mask = a modifier to a layer that "masks" out part of an image from showing(see my sig for a tutorial on layer masks)
Note that channels and layers can be named. This is just informational data so you can keep them seperate and organized.
For the invert, it is only available if you have a layer or a channel selected. If there is no selection, it has no way to know what to invert.