Several brushes in ZBrush insert 3D meshes into your object rather than sculpting it. Some of the default brushes are named IMM in the brush menu, but you can get dozens of others from sites like BadKing.com.au (check it out – they’re phenomenal).
IMM stands for Insert Multi Mesh, and such brushes can be created with several geometries under the hood. Imagine a brush that inserts buttons: there may be more than one button style one might need, and such styles could be stored in the same IMM brush. Very handy! Try it out with the IMM Parts brush: draw an object, enter edit mode, use the shortcuts B-I-A to select the brush and drag to insert a default button on your object.
To see the other brush styles, hit M. This brings up a pop-up menu with anything else this IMM brush has to offer. Pick another part and drag again to insert it.
Once dragged out, use the space bar to move the position of the inserted mesh. You can constrain the new mesh to its original size by using the CTRL key.
IMM is a huge topic though, and we’ve barely scratched the surface of what can be done with it. Check out the ZBrush docs for more details: