New Gimper question on Transparent layers

[StylinLP38]

Im confused in GIMP the difference between adding an ALPHA channel to a layer or choosing the ADD Transparent Layer to a layer.
Also, when you create a new Layer you have a choice between creating it with WHITE or Transparent. 
Im trying to grasp how to create many layers with a different over land textures so I can create fields, grass, woods, hills, gravel, roads. Its really confusing. 
Why would I choose ADD an ALPHA channel to an existing texture layer vs creating a new transparent layer?

[Blighty]

Add Alpha Channel = enable transparency
No alpha channel means there cannot be transparency
If you add a transparent layer then there is transparency ie there is an alpha channel
If there is an alpha channel, transparency can be set between 0 and 100%
0% = transparent; 100% = opaque

If a layer is white there are 2 possiblities
1) There is an alpha channel; but opacity (transparency) is set 100% so it is not transparent
2) There is no alpha channel so there cannot be transparency

Layer masks are used to control the transparency. Portions of a layer can be made transparent or semi-transparent; other portions of the same layer can be opaque.

[StylinLP38]

Ok, I see, there is 3 ways to add an Alpha channel to a layer.