A Voxel - short for volumetric pixel - is basically a single colored cube. All voxels of the model have the same size and orientation. A Voxel may not exist outside of a Matrix.
A Matrix is a 3-dimensional grid of voxels. Matrices have a maximum size of 1024x512x1024 but should be kept within 128³ to maximize performance. Larger models should be built out of several Matrices.
A Model is always the root object of your file. The model exists with no size or transform attributes and is a space for other objects like Matrices and Compounds. You can place as many objects in a model as you like (in fact it's technically limited to a maximum of 16 million, but you will never reach that).
Compounds enable you to organize your model. You can pack any amount of Matrices and other Compounds into a Compound and thus establish object hierarchies. Compounds will also speed up render performance and are useful for mesh export and modeling workarounds. If you are a Photoshop user then think of Compounds as Smart Objects. Compounds are distinguished from Matrices by a dashed Bounding Box.
The Bounding Box is a grid that delineates the size of an object, like the canvas in Photoshop. Voxels in a matrix cannot exist outside of the bounding box. Compounds will truncate the voxels of component objects that extend past the bounding box, but the voxels will still exist inside the compound.