A selection is what you make using the lasso or marquee tools. Anything inside the ring of "marching ants" will be affected by whatever adjustement you do next.
A mask "masks off" parts of a layer making it invisible.
Try this to help you get the hang of both:
* Open an image file
* Click the "new layer" icon at the bottom of the Layers palette.
* Rename the "background layer to (say) image.
* Drag this "Image" layer to the top (above the new empty layer)
* Fill the empty layer with white.
* Make the Image layer active and create a selection with the marquee tool (round or square) See the "marching ants"? That’s your selection.
* Now hit the "Layer Mask" icon at the bottom of the layers palette.
See how the mask has been created, based upon your selection. You can modify this mask by painting on it with the paintbrush, using black or white, depending whether you want to hide more of the image or reveal parts which are hidden.
Have fun!
Chris.