mike is exactly correct
i think another item you are confused about is this:
the two modes you are switching between are quick mask and selection, you are not using standard masking.
forget the word "mask" in "quick mask"
think of "quick mask" as another "selection" tool, or a selection-changing tool.

TRY THIS OUT, consider this a pictureless tutorial::::::
proceed like this:

open a picture
select something with the lasso
now hit the letter "q"
that puts you in quick mask mode, the marching ants will disappear and in their place will be red color over all the areas previously NOT selected by the lasso, everything included in your selection with the lasso will be unaffected, anything NOT included in your lasso selection will be red.

in other words:
you will "see" what was selected by the lasso, everything not selected will be "masked" by red.

now you can add to or subtract from your selection by painting with white or black over your picture while you are still in quickmask mode
so hit the letter "d".
that will make your two color swatches become black and white
you can toggle between black and white by hitting "x" as mike said.
wherever you paint with black will turn red in your picture which indicates that it is being removed from your selection.
wherever you paint in white, you will see the red color disappear as you are ADDING to your selection.
now if you hit "q" again you will return to regular selection mode.
you will see new marching ants.
everywhere that was not red in your picture while you were previously in the quickmask mode will now be included by the marching ants in this new selection (which has been modified by you while you were in quickmask mode)

feivel