The PDF Mask features offer a whole set of functionality to allow complicated image drawing to happen. Rather than just sticking an image on the page (over-writing whatever was underneath), Masks allow sophisticated use of transparency so that the image can be blended into the page and colours or areas can be wholly of partly transparent. There are 3 types of Image mask.
The simplest form of Mask is an ImageMask. This is a one bit image where each pixel is either transparent (and you can see what is behind it) or painted in the current colour. It is rather like a stencil (which lets paint through the holes but stops paint appearing elsewhere).
An ImageMask is easy to create – it is a 1bit XObject image which has a value of /ImageMask set to true. They are limited to a single colour however. So you can also have a proper Mask for more control.
A Mask allows you to specify an image which can contain transparent elements. The Mask is an object attached to the XObject. You can either specify a range of colours (which will become transparent in the image) or add a separate bitmap which can define each pixel as transparent or visible (a bit like an ImageMask but offering more flexibility).
Most flexible of all, you can have a soft Mask (SMask object) attached to the image. This does not just switch transparency on or off but lets you control how much of the image shows through – allowing fine grained transparency.
Masks are really powerful features – if you have not tried them before, they are well worth experimenting with. What do you use Masks for?