Greetings cartographers.

I have learned the basics of GIMP and wanted to follow a tutorial now ( http://www.cartographersguild.com/tu...mp-wilbur.html )
I am at the stage Bump Map, Creating additionnal layer masks at p.6 and am stuck. I spent1 hour on GIMP tutorial first but still don't understand what to do.

In GIMP there are files, images, layers, masks and channels all referring to similar things so that the instructions are confusing for me.
For me :
- a file and an image are the same thing, this is what I load with "Open file".
- a layer and a mask are the same thing too. I can load a file as layer and then it appears in the layer window. So I know how to create a layer with any file I have. If layer and mask are same things why different words are used ?
- I don't understand what a channel is and how it is different from a layer/mask. I see the channels window but it is empty (excepted for the RGB,alpha channels)

Now the instruction in the tutorial is : "Now we are going to create 2 additional layer masks. The first layer mask is simply a new version of our "Land Mask". In your channels dialogue click on the Land mask channel and then click on the Create Duplicate Channel Icon"

As I opened a file Land mask that I created some time ago, I have a file and an image called Land Mask. I have also a layer that is called Land Mask.
But I have NO Land Mask channel so I can click on nothing there. What should I do to have a "Land Mask channel" containing the Land Mask file ?
Also what does it mean to "create a layer mask" . Does it mean the same thing as "create a layer" ? But as I know how to "create layers", why suddenly jumping to "channel dialogues" ?

Thanks in advance for any explanation helping to unstuck me