Masking lets you create a stencil effect. You can cover an object with a masking object, so that the image only looks through the shape of the masked item. You can have many paths as masks and can have multiple masks on a page, although, multiple or complex masks could cause printing problems.
Making a mask:
Draw the item you want to mask
Now place on top of these objects your masking object (or mask)
Select both the objects and the mask.
Go to the Object menu and choose Clipping Masks, Make:
You can use your selection tools to move your mask or the objects around and change how the mask looks.