You’re in the wrong forum here, this one is about feature requests.
I would recommend that You peruse the Help-file »photoshop_cs4_help«, a downloadable pdf of which should be available here:
»Masking Layer« on page 319 might be helpful, otherwise please consider posing Your question in the Photoshop MacIntosh- or Photoshop Windows-forum.
But to give you and anyone Googling for an answer to this question a short answer, think of layers as those transparency sheets you see used on an overhead projector. When you create a new layer you just stack on another transparency sheet. If you paint on that layer it only effects that sheet.
Now masks are like masking tape in some respects. The masked area is prevented from being painted on or otherwise affected. The difference with masking tape is that Photoshop masks can actually have variable opacity, something masking tape can't do.
In Photoshop, the selected layer is only effected by the masked area. That is the relationship between them.