Look at the bottom of the toolbox. Up until now you have always worked in Standard Mode.  As you know, Standard Mode indicates the limit of the selection with a line of marching ants. Open any picture and select part of it using the rectangular marquee tool. Then switch to Quick Mask mode by clicking the right icon in the edit mode section at the bottom of the toolbox:   You will notice that in Quick Mask mode the selection appears clear while the unselected area appears to be protected by areas of transparent red plastic. 

It's important to understand that while you are in Quick Mask mode you are only modifying the mask that is floating above the picture (though it is not strictly speaking a separate 'layer') - you are not modifying the underlying picture at all. For example, you might try to draw some colored lines onto the picture using the paintbrush but you will not succeed - all that will happen is that you extend the masked (protected, red) area if your chosen color was dark or you will extend the selection (the clear area) if your chosen color was light. As you see, the actual colors used do not matter in Quick Mask mode - all that matters is whether the current foreground color is light or dark. If the foreground color is white, as here: , then your brushstrokes will completely erase the existing mask (assuming you have chosen a hard-edged brush). If the foreground color is black (you can switch the background and foreground colors by clicking the double-headed arrow in the above picture) then your brush strokes will apply a thoroughly impermeable mask that will completely protect that part of the picture from changes once you return to standard mode. If the foreground color is neither white nor black then your brush strokes will apply a mask that is somewhat permeable - the lighter the color then the more 'permeable' will be the mask and the more these areas will be affected by changes you attempt to apply to the picture after returning to standard mode. Now, read this paragraph again - it's important!

To get some practice in Quick Mask mode, let's use the picture below. I took it in Thailand on a canoeing trip with students of the Hong Kong International School. The stilt house is occupied by a family that farms collects swallows' nests from the caves of this island. Swallows' nests, made from the birds' saliva, are an expensive delicacy in many Asian countries.

Although the picture is already pretty good (I took it myself, after all), we'll try to give it a surreal feel as in the photo below. We want to apply a strong affect to the top of the picture, gradually weakening towards the bottom. This is a perfect reason to use the Quick Mask mode.

Copy the picture to your folder, then open it and switch to Quick Mask Mode. We'll use the Linear Gradient tool to create the mask. Before you use the tool, double-click the linear gradient tool and set the gradient to Black, White (make sure also that the tool is set to normal and 100% opacity). Now drag with the tool from the bottom of the picture to the top - holding down the Shift key while you do this will help to ensure that you drag vertically. In standard mode this would apply a gradient going from black to white but since we are in Quick Mask mode we are applying a mask that appears red at the bottom and clear at the top. This is what we want since the clear parts of the picture will be affected most by changes that we make after returning to standard mode.

Now return to Standard Mode so that we can apply the color effect. Notice that only the top part of the picture seems to be selected, according to the marching ants. But this is misleading - the marching ants cannot show the true nature of the mask so they just indicate which part of the picture is going to be affected most by modifications that we apply. I got the color effect as follows: Choose Image>Adjust>Hue/Saturation. We want to change the hue (teinte in French) of the sky but moving the hue slider has little effect because the sky almost white and therefore there is no color to change - the hue control works best for areas that are brightly colored. Turning on the colorize option (redéfinir, in French) changes that so the hue control affects the hue of all parts of the picture that are neither very light nor very dark (thus including the gray parts as well as the brightly colored parts). Turn on the colorize option, and then use the brightness control to make the sky gray rather than nearly white. NOW the hue setting should allow you to apply a nice color of your choice to the sky. I finished the picture by deselecting the mask (Ctrl-D) and then applying a Levels adjustment (Ctrl-L) to make sure that the picture contained a little pure black and pure white. Save your work (full size).

Note that a similar effect can be produced without using a mask by simply setting the linear gradient tool to foreground, transparent and then dragging from top to bottom. The result is shown below but I don't like it as much as the effect above because the purple foreground color has washed out details in the clouds and in the stilt house. You do not need to try to reproduce the effect below but you should spend a few minutes experimenting with the various gradient tools if you have not already done so.

You should get some more practice with the Quick Mask mode, so try changing the first picture below into the second (you don't need to add the word 'sample'). The picture shows a typical resident of San Francisco, California.

I did this in a similar way to the Thailand picture, but this time I used the Radial Gradient tool to put a mask over his/her face before using Image>Adjust>Hue/Saturation or just Image>Adjust>Desaturate to take the saturation of the unmasked areas progressively down to zero. Save your work.