As you've asked so nicely Steve...
very very simple (and possibly not the 'correct' way of doing stuff) but it works for me!!!
1. open your image that you want to make adjustments on
2. cmd + J to duplicate the layer
3. on the uppermost layer (the one you've just created) apply whatever effect you want - this will apply the effect to the whole image
4. this is the layer mask bit.... click on the rectangle with the circle in it at the bottom of the layers pallet - should be at the very bottom of PS on the RHS - it's on the same row as the trashcan.
5. hit 'B' to activate your brush tool (and change size, softness, opacity etc and shape at this point if you want to)
6. on the far left there is a vertical tool bar - click on the tiny squares that are black and white (not the larger ones next to them) this will turn your brush into black or white
7. I can never remember which hides and which reveals - paint anywhere on your image with whichever brush comes up (black or white), if the effect you've applied gets brushed away, you've got the right colour brush, if not click on the right angled arrows just above the larger black & white squares to swap them (from black to white or white to black), and bingo - add and remove the effect as you like - and if you make a mistake, swap the colour back and you can reapply it!
Easy!
Let me know if this helps
And as I said, it may not be the correct way (and there are normally loads of different ways in PS) but this is the simplest way I've found and it works for me...
You can also do it so the effect gets brushed in, just hold down ALT when opening your layer mask, and cmd + I to invert it