Another way of joining objects is to use clips and masks. These features are used to determine which parts of an object are visible. Clips define what areas of another object are fully visible. Technically speaking, Inkscape takes the top object's path in and clips all the paths below it (in the selection) to the shape of the top path.
When you use a mask it visually crops objects with transparent areas to become fully transparent in the masked object, white areas become fully opaque, and all other colors translate into different levels of opacity in the masked object. To make sure this is clear, we'll do a few exercises.