Hi,
I'll try and answer a few of your questions.

The difference between 2- and 6-sided mapping is this:

2-sided mapping tries to map the whole model. You can't select a certain part of the model, only the whole thing. In an attempt to cover all sides it will create either a - front and back map - or a - left and right map - or a - top and bottom map -

6-sided mapping allows you to map individual parts of the model. This is what is useful in most situations 'cos it allows you to avoid the really bad stretching on the sides of models that you get when using 2-sided mapping.

As to what mapping does:

I am not quite sure what you mean with your description but as far as I know mapping has to do with creating a 2d map of your 3d model. With 6-sided mapping you're deciding from what direction you want to view a certain part of the model to get an optimal view of that section.

To stop the leaking of colours you should make sure that there is always a little border around the area you mapped in the colour of the part you are mapping. So if you want to colour one of the squares white you should leave a few pixel wide border of white around the square.

To make sure your skin isn't distorted you have to make sure that if something is a square the map you have created is also square. Make sure the proportions are correct.

Hope this helps.