A mask is a vector path which allows part of an object to show while blocking out the rest effectively making part of it transparent. A mask in an illustration program is functionally equivalent to a clipping path in an image editor.
1. Take for example a photo. You can create a mask which will knockout the background.
2. Using the Pen, draw a path around the bird in the photo.
3. This is a view of the path only.
4. When the path is made into a mask and combined with the background image, the background becomes transparent.
5. Masks can be made to create many kinds of effects. A type mask can be combined with a background photo.
6. This is the mask created by converting type to curves.
7. This is the type mask when combined with the background image.
More Vector Graphics: 
