Trying to figure out the logic behind the new T3 BS3 classes.

Hi,

I'm trying to customize a blank template for BS3, to be able to accommodate the wishes of my client.
Doing this, I create new layout files for my template, but can't figure out the logic of the use of classes in the new BS3.

Is there a explanation to the classes defining the widths of the columns, floating around somewhere?
For instance - what are the differences to the classes "col-xs-somewidth" and "col-sm-somewidth"? I see both these classes being used in same div's - but am not sure if "xs" means "extra small" or if "sm" is small" - and why to use both in same div? I'm sure it's related to the responsiveness of the template, but not sure why only 2 classes are mentioned then....

Hope someone could clarify this for me

/Zorroson