A mask might appear to be just a simple tool for covering something up, but Adobe Illustrator's masking tools offer far more than one basic use. Along with the Compound Path, Illustrator's Clipping and Opacity Masks can take your illustration and design onto a new level of detail, precision, texture and shape. To know when to use each tool, you simply need a clear understanding of what each one does.
Briefly, a Compound Path takes an object and puts a hole through it; a Clipping Mask and a Layer Clipping Mask both mask off everything outside a specific shape, making a silhouette with the artwork, colours or patterns within it; and an Opacity Mask enables you to gradient the transparency of an object, fading it from 100 per cent opaque to completely invisible. Once you've learned these four tools and know how to create masking boundaries for your artwork, you'll be able to continually adapt the technique, breaking boundaries with your design work.