Masks

Masks are used to hide parts of the layer without altering layer's contents.

There are two types of masks available:
Layer Mask - applies to a single layer
Clipping Mask - a layer becomes a mask for a number of layers clipped onto it

(!) Both mask types use Alpha channel, meaning that masks use transparency of their contents to hide or show layers' contents.

Controls
Use dedicated buttons and layers options drop-down.
A few extra controls include:
Disable / Enable Layer Mask - Shift+Click on mask
Move Layer Mask to another layer - Click & Drag mask to a destination layer
Copy Layer Mask to another layer - hold Ctrl+Click & Drag mask to a destination layer.
Clear Layer Mask - clicking on dedicated clear button multiple times will alternate between clear and fill.